Implement payment delegation, compute budget estimation, dispute mechanism, and protocol fee

[brite-side0]

Summary

• Issue Add payment delegation — allow a third party to pay on behalf of a payer #315 — Payment delegation: set_delegation(invoice_id, on_behalf_of, delegate) grants single-use authorization; pay_invoice_delegated(executor, invoice_id, amount, nonce, on_behalf_of) lets a third party fund on behalf of another address. Both payer and executor are recorded; a DelegatedPayment event is emitted. Delegation is consumed on first use (replay-proof).

• Issue Add compute budget estimation utility for all contract functions #316 — Compute budget estimation: estimate_compute(function_name, recipient_count) returns { instructions, mem_bytes, read_entries, write_entries } for all public functions. Warning event emitted above 80% of the Soroban instruction limit. Adds COMPUTE_BUDGETS.md with measured counts for 1/5/20 recipients and a new CI workflow (.github/workflows/compute-budget.yml) that posts a budget table on each PR.

• Issue Add invoice dispute mechanism with arbitration window #325 — Invoice dispute mechanism: raise_payer_dispute(invoice_id, payer, reason_hash) callable by any payer within 48 ledgers of full funding — sets disputed = true and blocks release. resolve_payer_dispute(invoice_id, admin, outcome) (admin only) either releases (Approved) or refunds (Refunded). expire_dispute(invoice_id) auto-releases after 72 ledgers if unresolved. get_dispute() returns the DisputeRecord. Events: DisputeRaised, DisputeResolved, DisputeExpired.

• Issue Implement protocol fee distribution to treasury address #326 — Protocol fee distribution: set_protocol_fee(admin, rate_bps, treasury) configures a fee (0–500 bps, default 0 = disabled). Fee is deducted atomically from release amount and transferred to treasury inside _release_full before recipient payouts. get_fee_config() returns { rate_bps, treasury }. FeePaid event emitted on each release.

Test plan

• set_delegation + pay_invoice_delegated: delegation grant → delegated pay → confirm payer recorded as on_behalf_of, replay attack (second call) panics with "no delegation authorization"
• raise_payer_dispute within 48-ledger window → release blocked → resolve_payer_dispute(Approved) releases funds
• raise_payer_dispute → resolve_payer_dispute(Refunded) → payers receive refunds
• raise_payer_dispute → wait 73 ledgers → expire_dispute releases to recipients
• set_protocol_fee(100, treasury) → release → treasury receives 1% → recipients receive 99%
• set_protocol_fee(0, treasury) → fee disabled (no-op path)
• estimate_compute("release", 20) → returns values under 100,000,000 instructions

Closes #315
Closes #316
Closes #325
Closes #326

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