feat: add decentralized invoice rating oracle (#877)

[jacobjacobb4242]

Issue #877 — Add decentralized invoice rating oracle

Adds a new rating_oracle_ext.rs module implementing a two-channel
decentralized rating system: payer-submitted ratings and admin-registered
oracle node ratings.

Core functionality

Oracle node management (admin only):

• register_rating_oracle(admin, oracle) — add an oracle node.
• remove_rating_oracle(admin, oracle) — remove an oracle node.
• is_rating_oracle(oracle) — membership check.

Payer ratings:

• rate_invoice(payer, invoice_id, score, comment)
  • Score 1–5; optional comment ≤ 280 bytes.
  • Caller must have paid the invoice at least once.
  • Each payer may rate once per invoice.
  • Scores are aggregated (sum + count) for efficient average calculation.

Oracle ratings:

• push_oracle_rating(oracle, invoice_id, score) — oracle node pushes a score (1–5). Latest push overwrites; useful for live credit-score feeds.

Read-only:

• get_invoice_rating(invoice_id) → InvoiceRatingSummary { score_sum, count, raters }
• get_oracle_rating(invoice_id) → Option<u32>

Events

Topic	Data
(split, ora_add)	oracle
(split, ora_rem)	oracle
(split, inv_rat, invoice_id)	(payer, score, comment)
(split, ora_rat, invoice_id)	(oracle, score)

Storage

Local RatingOracleKey enum:

• RatingSummary(invoice_id) — aggregate struct in persistent storage.
• OracleScore(invoice_id) — latest oracle score in persistent storage.
• OracleNodes — registered oracle node list in instance storage.

New type

InvoiceRatingSummary { score_sum: u64, count: u32, raters: Vec<Address> }

Tests

9 unit tests:

• Payer can rate invoice
• Non-payer cannot rate
• Double rating rejected
• Invalid score (0) rejected
• Oracle can push rating
• Non-oracle cannot push
• Oracle rating defaults to None
• Oracle rating overwritten on re-push
• Register and remove oracle
• Multiple payers aggregate correctly

Files changed

File	Change
contracts/split/src/rating_oracle_ext.rs	New
contracts/split/src/lib.rs	Added mod rating_oracle_ext;

Closes #877

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