feat: resolve issues #835 #836 #837 #838

[ahmadrabiumustapha]

#835 – multi_currency.rs: add fee-asset consistency policy

• Add fee_asset field to MultiCurrencyConfig (defaults to XLM)
• Add FeeAssetValidation enum and validate_fee_asset() method
• Add FeeCalculation struct and collect_fee() method — fees are always denominated in fee_asset regardless of settlement token
• Add tests for XLM, USDC, and EURC settlement currencies plus zero-amount and policy-coverage tests

#836 – rollback_tests.rs: multi-currency and cross-contract coverage

• Add multi-currency rollback tests: USDC-like refund, RolledBack status, EURC after-window rejection, two-currency independence
• Add cross-contract rollback tests: ip_registry intact after rollback, no funds stuck in contract, IP record stays consistent, IP can be reused immediately after rollback

#837 – registry.rs / architecture.md: document role and relationship

• Add module-level doc comment to registry.rs explaining it is a local read-only proxy for ip_registry (not a standalone registry)
• Add 'registry.rs — Local Registry Helper' section to docs/architecture.md with ASCII relationship diagram and table

#838 – handlers.rs: wire commit_ip to Soroban RPC

• Add api-server/src/soroban_rpc.rs with SorobanRpcClient trait, LiveSorobanRpcClient (reqwest + env vars), MockSorobanRpcClient, and map_rpc_error_to_status() (400 / 404 / 503 / 500)
• Wire commit_ip handler to SOROBAN_CLIENT Lazy static; remove TODO
• Add 503 response code to utoipa path docs
• Register soroban_rpc module in main.rs and lib.rs
• Add commit_ip integration tests in integration_tests.rs

chore: update .gitignore

• Exclude test_snapshots, snapshots, jest coverage, fuzz queue/hangs, node_modules, *.wasm, deployment artefacts, api-server/Cargo.lock, .stellar/, log/tmp files, vrickish.md
• closes Add multi-currency fee-asset consistency checks #835
• closes Add rollback test coverage for partially-completed multi-currency swaps #836
• closes Document the registry.rs role and its relationship to ip_registry #837
• closes Wire commit_ip handler to the actual Soroban RPC call #838

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