fix(#838,#837,#836,#835): wire commit_ip RPC, document registry.rs, a…

[ahmadrabiumustapha]

…dd rollback+fee-asset tests

#838 - Wire commit_ip handler to Soroban RPC

• Add api-server/src/soroban_rpc.rs with commit_ip client, SorobanError enum, and error classifier (maps contract error codes to typed variants)
• Replace TODO stub in handlers::commit_ip with full soroban_rpc::commit_ip call; map every SorobanError variant to the correct HTTP status code (400 ZeroHash, 409 Conflict duplicate, 403 Forbidden, 410 Gone, etc.)
• Register soroban_rpc module in lib.rs and main.rs
• Add commit_ip integration tests: payload validation, zero-hash guard, hash-length check, error→HTTP mapping table, success type check

#837 - Document registry.rs role and relationship to ip_registry

• Add module-level doc comment to registry.rs clarifying it is a cross-contract adapter, not an independent registry
• Add '🗂️ registry.rs' section to docs/architecture.md with a function table and ASCII call-graph showing the adapter→ip_registry relationship

#836 - Multi-currency and cross-contract rollback tests

• Add multi-currency rollback tests (XLM, USDC, EURC) in rollback_tests.rs confirming the 90% buyer refund works for every settlement token
• Add contract-balance check: no residual buyer funds after rollback
• Add cross-contract rollback tests: IP ownership stays with seller, ActiveSwap index is cleared (re-listing allowed), RolledBack ≠ Cancelled

#835 - Fee-asset consistency for multi-currency swaps

• Document fee-asset policy in multi_currency.rs module header with rationale, policy rules, and a scenario table
• Add fee_asset field to MultiCurrencyConfig (defaults to XLM)
• Add validate_fee_asset() (panics with InvalidFeeAsset on mismatch)
• Add is_valid_fee_asset() boolean predicate for off-chain callers
• Add InvalidFeeAsset = 66 to ContractError in lib.rs and errors.rs
• Add 11 fee-asset tests covering XLM/USDC/EURC accepted and rejected combinations, reconfiguration, and the settlement-vs-fee independence invariant

.gitignore - expand to exclude wasm outputs, .stellar/, node_modules/, coverage/, criterion/, fuzz corpus subdirs
closes #835
closes #836
closes #837
closes #838