feat(app): /attester read-only payout-account checks

[jdjioe5-cpu]

What and why

Closes #42.
Replaces the /attester placeholder stub with a fully functioning, read-only payout-account verification page. Attesters can verify institutional payout accounts before onboarding or publishing registry records, strictly matching the verdicts of kinlock-registry/scripts/check-attester-readiness.ts without asking for secret keys, seed phrases, or transaction signatures.

Changes

• lib/attester/strkey.ts (new): RFC 4648 Base32 + CRC-16 XModem validator for Stellar Ed25519 public keys (G-addresses). Pre-validates user inputs before any network calls, rejecting invalid prefixes, short inputs, and corrupted checksums.
• lib/attester/readiness.ts (new): Core evaluation engine that categorizes Horizon accounts into three explicit verdicts:
  1. Account existence (account.exists);
  2. Native XLM float vs reserve requirement (>= 2 XLM);
  3. USDC trustline status: strictly distinguishes missing (absent) vs unapproved (unauthorised) vs full limit (no_room) vs authorised with room, showing the asset code and configured issuer.
• app/attester/useAttesterChecker.ts & AttesterChecker.tsx (new): Client component and decoupled state hook with loading states, role="status", WCAG AA contrast, and zero inline strings.
• app/attester/page.tsx: Replaced PageStub with AttesterChecker, configured with export const dynamic = "force-dynamic".
• messages/en.json: Added all user-visible strings under pages.attester.*.
• README.md & ROADMAP.md: Documented /attester in the routes table, advanced M3-12 to DONE, and added Changelog entry.

Hard rules / invariants touched

• Hard Rule 3: Chain is truth. Reads live ledger state from Stellar Horizon.
• Hard Rule 4: Non-custodial. Zero private keys, seed phrases, or transaction submission.
• Hard Rule 5: No personal data logged or persisted.
• Hard Rule 11: Country-agnostic core. Asset code and issuer are derived dynamically from config/registry; zero country or currency literals.

Tests

Added lib/attester/readiness.test.ts with 7 test cases covering all verdict branches against Horizon fixtures.
Ran all verification commands cleanly:

pnpm lint
> biome check .
Checked 88 files in 23ms. No fixes applied.

pnpm typecheck
> tsc --noEmit

pnpm test
✓ lib/i18n/inline-strings.test.ts (30 tests)
✓ lib/country-agnostic.test.ts (62 tests)
✓ lib/i18n/format.test.ts (11 tests)
✓ app/claim/[id]/claim-logic.test.ts (7 tests)
✓ lib/receipts/parse.test.ts (10 tests)
✓ app/payee/payee-logic.test.ts (8 tests)
✓ lib/security/csp.test.ts (5 tests)
✓ lib/i18n/messages.test.ts (3 tests)
✓ lib/attester/readiness.test.ts (7 tests)
✓ lib/locks/refund.test.ts (6 tests)
✓ lib/config.test.ts (4 tests)
✓ lib/indexer/index.test.ts (4 tests)
✓ lib/sdk.test.ts (3 tests)
✓ app/request/schedule.test.ts (4 tests)
✓ lib/claim-links/claim-links.test.ts (6 tests)
✓ app/send/send-logic.test.ts (8 tests)
Test Files  16 passed (16)
     Tests  178 passed (178)

pnpm build
▲ Next.js 16.3.8 (Turbopack)
✓ Compiled successfully in 447ms
✓ Generating static pages using 14 workers (5/5) in 423ms
Route (app)
├ ƒ /attester
...

Side-by-Side Parity Evidence against check-attester-readiness.ts

1. Existing Funded Account (GCLMHF7LAR34TSELDNEREYPTNE4K7MMESPS62WXE7ZDLPWIJUF3MC5QM):

• check-attester-readiness.ts:

  ✓ native XLM float: 9999.9647224 (need >= 2)
  ✓ USDC trustline authorized: true
  ✓ USDC trustline room: limit 9223372036854775807 > balance 80000000
  Account and trustline checks pass.
  

• /attester Readiness Report:
  • Account: pass (exists on testnet)
  • Native XLM: pass (9999.9647224 XLM >= 2 XLM)
  • USDC trustline: pass (authorised with room)
  • Total problems: 0

2. Unfunded / Missing Account (GCC7BYEDK2RJKHKRRY6QVEUYNRBAYRCTU76MQFPUGD7D3AO6G5XITMPJ):

• check-attester-readiness.ts:

  ✗ account GCC7BYEDK2RJKHKRRY6QVEUYNRBAYRCTU76MQFPUGD7D3AO6G5XITMPJ does not exist on testnet
  ✗ no USDC trustline (USDC:GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5) on GCC7BYEDK2RJKHKRRY6QVEUYNRBAYRCTU76MQFPUGD7D3AO6G5XITMPJ
  2 problem(s) found.
  

• /attester Readiness Report:
  • Account: fail (does not exist on testnet)
  • Native XLM: null
  • USDC trustline: fail (absent)
  • Total problems: 2

Docs / ADR

• Updated README.md
• Updated ROADMAP.md (M3-12 -> DONE)

Roadmap

• Row M3-12 marked DONE
• Changelog entry added: yes
• Progress snapshot refreshed: yes

Assumptions and open questions

None.