implement CEP-8 Phase B Lightning rails (NWC and zaps)

[harsh04044]

Summary

Phase B of CEP-8 (#100) replaces the deterministic fakes with real Lightning rails behind the PaymentProcessor and PaymentHandler traits Phase A froze. No trait, middleware or registration-entry-point changes: a rail is a plug-in.

Two rails, in this order. NWC (NIP-47), both sides of the wire: a processor that issues a BOLT11 invoice (make_invoice) and verifies settlement (lookup_invoice polling or payment_received notifications), and a handler that pays (pay_invoice). NIP-57 zaps, server side only: an invoice from an LNURL-pay callback carrying a kind 9734 zap request, verified against a kind 9735 receipt. LNbits, the third rail listed in #100, is dropped.

Settled before planning:

• CI does not drive a real wallet. Relay-level (NWC) and HTTP-level (LNURL) mocks are the merge gate; ts-sdk gates nothing on a wallet either. A mock wallet service over MockRelayPool drives the real client code rather than stubbing it. Real-wallet tests ship #[ignore]d and environment-gated, run by hand before each rail merges.
• No zap handler. NIP-57 obliges the payer only to pay a BOLT11 invoice, and the provider publishes the receipt. Both rails issue pmi: bitcoin-lightning-bolt11 and handlers are keyed by PMI, so a zap offer already routes to the NWC handler, as ts-sdk's zap-integration.test.ts does.

Key notes:

• nostr 0.44.4 ships nips::nip47 and nips::nip57 behind flags nostr-sdk already forwards, so no NIP-47 crypto or zap-request construction is written here. The nwc crate is not used: its relay pool is private, so nothing on top of it is testable without a live wallet.
• RelayPoolTrait has no unsubscribe, so ts-sdk's subscription-per-request would leak one live REQ per payment. Each rail client holds ONE subscription and correlates in process by request event id.
• Wallet responses are parsed permissively, ts field for field, not through nostr's strict response structs. On a money path a missing optional field must not turn a paid invoice into a failed verification.
• Bounds are the rail's job: create_payment_required runs in the middleware's unbounded, cancellation-blind initiation phase; verify_payment must observe params.cancel or its poller outlives the timeout; the client engine gives handle() neither a timeout nor a cancellation token.
• Two safe-direction divergences from ts-sdk, invisible on the wire: NIP-47 notifications are decrypted by kind (23196 is NIP-04, 23197 is NIP-44), and notification-mode verification falls back to polling instead of throwing when no payment_hash is cached.
• Features off by default: nwc = ["nostr-sdk/nip47"] adds no new crate; zaps = ["nostr-sdk/nip57", "dep:reqwest"] adds the only new dependency in Phase B, because a zap fetches its invoice over HTTP.

Spec: contextvm-docs/src/content/docs/reference/ceps/cep-8.md. It is rail-agnostic; the only normative rail text anywhere is CEP-21's line that pay_req is a BOLT11 invoice string.
TS SDK reference: sdk/src/payments/nip47/, nip57/, processors/, handlers/
Phase A: #100 (11 PRs, #101 to #108 and #111 to #114, all merged)

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PR B1: NIP-47 client core + mock wallet (rail infrastructure)

• Cargo.toml - nwc = ["nostr-sdk/nip47"], off by default, no new crate
• src/payments/nip47/uri.rs - parse_nwc_uri, accepting the pathname form NostrWalletConnectURI::parse rejects and ts-sdk accepts
• src/payments/nip47/types.rs - permissive {result_type, error, result} envelope plus all-optional per-method results mirroring ts nip47/types.ts; sats_to_msats with checked_mul and a non-positive-amount rejection
• src/payments/nip47/client.rs - NwcClient over Arc<dyn RelayPoolTrait>: one subscription, one reader task, waiter map keyed by request event id, response_timeout per call, Lagged handling, notification decode by kind, fetch_info_notification_types from kind 13194
• src/payments/nip47/mock_wallet.rs behind test-utils - scripted wallet over a linked MockRelayPool (answer, error, silence, delay, malformed) plus a settle trigger publishing payment_received
• Tests: round trip for the three methods; uncorrelated and stranger-signed responses ignored; silence times out; a wallet error is not an Ok; a minimal settled result and an unknown error code both parse; both notification kinds decode; Lagged loses nothing; concurrent requests correlate independently; URI parsing both forms

PR B2: NWC payment processor (server side of rail 1)

• src/payments/processors/ln_bolt11_nwc.rs - LnBolt11NwcPaymentProcessor for bitcoin-lightning-bolt11
• create_payment_required - make_invoice in msats with expiry, invoice to payment_hash LRU, configured ttl (the wallet's expires_at is ignored, ts parity), bounded by response_timeout
• verify_payment - in-flight dedup by pay_req (shared future over Arc<Result<..>>, since PaymentError is not Clone); ts backoff schedule floored at poll_interval with jitter; NOT_FOUND pending, other wallet errors fatal; expired and failed fatal; settled on state == settled or settled_at > 0; receipt is payment_hash or ts's settled:<event_id>[:<settled_at>] fallback
• Notification mode - lazy OnceCell auto-detection from the info event, polling fallback when no payment_hash is cached
• Cancellation - select! on params.cancel in every wait; a cancelled verify returns Err, never an empty Ok
• LnBolt11NwcPaymentProcessorOptions, non-exhaustive, ts defaults (300 s ttl, 1500 ms poll, 60 s timeout, 5000 in-flight, 10000 hash cache), injectable pool and client
• Tests: the four ts unit tests ported; cancelled verify stops the poller; a hung wallet fails create_payment_required inside response_timeout; terminal states fatal and NOT_FOUND not; the fallback receipt; notification mode polls instead of erroring; overflow and non-positive amounts rejected before any wallet call; no log line contains the invoice
• Mutation-test the money paths (cancellation arms, settlement predicate, NOT_FOUND branch, dedup claim)

PR B3: NWC payment handler + wiring + docs + e2e (rail 1 capstone)

• src/payments/handlers/ln_bolt11_nwc.rs - LnBolt11NwcPaymentHandler, pay_invoice bounded by its own response_timeout; the spending decision stays with payment_policy
• tests/payments_nwc_e2e.rs - real client and server transports over one MockRelayPool group, a mock wallet on each side, both lifecycles
• tests/payments_nwc_real_wallet.rs - #[ignore]d, environment-gated, mirroring ts nwc-integration.test.ts and nwc-paid-capability-e2e.test.ts
• docs/payments.md - the shipped NWC rail, its options, and its operational notes (a late settlement is money out with no execution; the wallet is a trust boundary; rate limiting is still upstream's job); correct the paragraph listing LNbits as planned
• README.md, Cargo.toml test entries, CHANGELOG.md
• Tests: both lifecycles end to end including -32043 while verifying; a declining payment_policy stops the wallet call and synthesizes -32000; a never-settling wallet ends at the TTL with no acceptance; a pay_invoice error leaves the request pending and synthesizes nothing; the handler's own timeout bounds a silent wallet
• Real-wallet run executed and recorded in the PR, invoice redacted

PR B4: LNURL-pay client + zap event helpers (zap infrastructure)

• Cargo.toml - zaps = ["nostr-sdk/nip57", "dep:reqwest"]; reqwest optional, default-features = false, features = ["rustls-tls", "json"] (rustls is already in the tree; no OpenSSL). MSRV 1.88 verified across its tree as a gate item
• src/payments/nip57/lnurl.rs - parse_ln_address (LUD-16) and an injectable LnurlPayClient trait so the rail is testable without HTTP, with a reqwest-backed default that is https-only, timed out, body-capped and does not follow cross-scheme redirects (the callback URL is remote-controlled input)
• src/payments/nip57/zap.rs - kind 9734 zap request via ZapRequestData plus anonymous_zap_request, and bolt11_from_zap_receipt
• src/payments/nip57/mock_lnurl.rs behind test-utils - scripted pay params, invoice, error, allowsNostr: false, out-of-range amounts
• Tests: LUD-16 parsing including ts's four rejections; the well-known URL built as ts builds it; the callback carries amount and nostr and returns pr; a reason-only response errors with the reason; non-https and oversized bodies rejected; zap request tags in ts's order; receipt bolt11 extraction

PR B5: Zap payment processor + wiring + docs + e2e (rail 2 capstone)

• src/payments/processors/ln_bolt11_zap.rs - LnBolt11ZapPaymentProcessor for bitcoin-lightning-bolt11
• create_payment_required - fetch pay params, reject an endpoint without zap support, enforce minSendable and maxSendable, sign the zap request with the processor's ephemeral key, request the invoice, record {expected_zapper_pubkey, since, amount_msats} keyed by the invoice, return _meta: {"rail": "nip57"}
• verify_payment - one subscription for kind 9735 from the expected zapper since the recorded timestamp, resolve on a bolt11 tag equal to pay_req, return _meta: {"zap_receipt_event_id": ..}; in-flight dedup by pay_req; stateless fallback re-fetches pay params; cancellation and HTTP bounds as in B2
• tests/payments_zap_e2e.rs and tests/payments_zap_real_wallet.rs (the real-wallet zap test pays with the NWC handler)
• docs/payments.md - the zap rail, its options, and three non-obvious facts: the provider publishes the receipt to the relays named in the zap request, so the configured relay set must be one it will publish to; the provider's nostrPubkey authorizes receipts, so it is a trust boundary; there is no client-side zap handler, and why
• README.md, Cargo.toml test entries, CHANGELOG.md
• Tests: issue and verify with the receipt published onto the mock relay; a receipt for another invoice or from another pubkey does not settle ours; out-of-range amounts fail before the callback; allowsNostr: false fails with ts's message; cancellation returns Err; the stateless fallback verifies; full lifecycle with the NWC handler paying the zap invoice
• Real-wallet run executed and recorded in the PR, invoice redacted

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Exit criteria

• Both rails shipped and feature-gated; --all-features and --no-default-features green, MSRV 1.88
• docs/payments.md documents both rails and no longer describes Phase B as deferred
• Both real-wallet test pairs executed and recorded
• CHANGELOG.md carries a rails entry per PR

Out of scope: LNbits (listed in #100, deliberately dropped); any third rail; a client-side zap handler; changes to the Phase A traits, middlewares or entry points; multi-PMI payment_required fan-out and direct_payment / change, which stay out for ts-sdk parity as in #100.

Not Phase B's, but load-bearing once real money moves: rate limiting. Every successful offer still spawns one unbounded detached verification task, and with a real rail each holds a wallet connection and polls a wallet. max_pending_payments and the authorization store caps bound state, not tasks. Worth its own issue.

[nisargpatel7042lva]

@harsh04044 I have completed PR B1. Could you please review it to check if everything is in order? Let me know if anything needs to be added or changed. In the meantime, I am starting work on PR B2.