Feat: invoice prioritization, creator fee tiers, invoice metadata & confidential payment tracking (#918, #919, #920, #921)

[kossyomma-beep]

Summary

Adds four new contract features, each in its own module under contracts/split/src/ with a separate #[contractimpl] block. Changes to lib.rs are
limited to module registration, and new events are added to events.rs.

#918 — Invoice prioritization (priority.rs)

• PriorityTier (Low / Normal / High / Critical) derived from a 0–1000 score, with resource-allocation weights (1/2/4/8)
• InvoicePriority record per invoice; per-tier queues kept sorted by score (desc), then invoice id (asc)
• Validation: score range, open invoice status, creator/co-creator or admin auth; Critical tier is admin-only
• Entry points: set_invoice_priority, clear_invoice_priority, get_invoice_priority, sort_invoices_by_priority, get_next_priority_invoice,
  get_priority_queue, prune_priority_queue, tier lookup/weight helpers

#919 — Fee tier system for creators (creator_fee_tiers.rs)

• Versioned CreatorFeeTierConfig of up to 10 CreatorFeeTiers (zero-based start, ascending thresholds, non-increasing fees, 10% cap)
• Binary-search tier lookup on the creator's existing lifetime volume
• calc_creator_fee returns gross / fee / net at the tier rate; falls back to the platform fee when no tiers are configured
• Tier tracking via sync_creator_fee_tier (emits an event on tier change); single-tier rate updates and clearing

#920 — Invoice metadata storage (invoice_metadata.rs)

• MetadataEntry key-value store per invoice with per-key versioning, author and timestamp
• Validation: keys 1–32 chars [a-z0-9_.-], values ≤ 256 bytes, ≤ 20 entries per invoice; creator/co-creator writes only
• Per-key storage plus an index, so single-key reads/writes touch one ledger entry
• Set / batch set / remove / clear / permanent lock; get, get entry, has, list keys, get all

#921 — Confidential payment tracking (confidential_tracking.rs)

• Amounts are masked as Pedersen commitments over BLS12-381 G1 (reusing the contract's pinned generators); no plaintext amount is stored or emitted
• Zero-knowledge proof of knowledge of the opening (Fiat–Shamir Schnorr/Okamoto), bound to invoice and payer to prevent replay
• Homomorphic aggregate commitment lets the creator prove the invoice total without revealing individual amounts
• Encrypted amounts stored as bounded opaque ciphertext with a SHA-256 integrity digest
• Hash-chained per-invoice audit trail with full-chain verification
• Queries: record / opening re-verification, summary, payer list, paged audit trail

Events

pri_set, pri_clr, cfee_cfg, cfee_chg, meta_set, meta_del, meta_lck, ctrk_rec, ctrk_ver

Tests

Unit tests are included in each new module (tier boundaries, sorting, tier lookup and validation, metadata validation, ZK proof verification and tamper
detection, homomorphic aggregation, audit chain integrity).

Notes

• The fee tiers (Implement fee tier system for creators #919) are not yet applied in the release fee path, and confidential tracking (Implement confidential payment tracking #921) does not move funds. Both are self-contained and
  can be wired in later.

Closes #918
Closes #919
Closes #920
Closes #921

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