[sim3onkiel]

Closes #1499

Summary

Documents which Horizon and Soroban RPC providers and versions SplitNaira supports, and adds a bounded, read-only compatibility smoke suite.

Changes

• backend/src/rpcCompat.ts: compatibility matrix, semver minimum check, and bounded smoke runner
• backend/src/rpcCompat.test.ts: automated tests
• backend/package.json: new test:rpc-compat script
• docs/runbooks/rpc-compatibility.md: supported providers/versions, guardrails, how to run, how to add a provider
• docs/runbooks/README.md and CHANGELOG.md: index entry and changelog

Guardrails

• Read-only: no keys, signing, or transaction submission, so security and financial invariants are untouched
• https-only endpoints, rejected before any request is made
• Per-request timeout (default 5000 ms) and a max of 10 endpoints per run
• Sequential checks

Tests

• Success: healthy Soroban RPC, Horizon at the exact minimum version
• Failure: version below minimum, unhealthy RPC, HTTP 500, version not reported, timeout, non-https URL
• Boundary: inclusive minimum version, invalid version strings, exactly the max endpoints vs. one over
• Live smoke is opt-in (RPC_COMPAT_LIVE=1) so CI stays offline and deterministic

Validation

• npm run test:rpc-compat -w backend
• npm run lint -w backend
• npm run format:check -w backend
• npm run build -w backend
• npm run test -w backend

Environment-dependent steps

The live smoke needs outbound HTTPS to the public Stellar endpoints:
RPC_COMPAT_LIVE=1 npm run test:rpc-compat -w backend