feat(relay): add live-reloaded file sources and state endpoint

[Kaonael]

Summary

Add --sources-file to select Relay namespaces from a JSON file, keeping discovery as the default. Both modes implement an internal NamespaceSource stream and share a namespace membership reconciler. File mode polls for updates; discovery follows model-registration events and periodically refreshes namespaces through DRT. Explicit discovery namespaces remain fixed.

Reload namespace selection without restarting Relay, preserve unchanged watches, and retain applied membership on source errors. Retry failed watch setup independently of source updates. Support an optional connectionRevision guard through DYN_RELAY_CONNECTION_REVISION.

Expose mode, connection revision, listener readiness, and desired/applied source revisions at /engine/state through the existing engine route registry. The source count represents selected namespaces, including those without matching endpoints. Changes are limited to Relay, its Python bindings, documentation, and tests; runtime, operator, and gRPC contracts are unchanged.

Validation

• Relay Rust suite: 217 tests passed, including dynamic DRT namespaces, endpoint filters, source failure retention, and file reload.
• Built the native Python extension from this branch.
• Full Relay Python suite: 104 passed, zero skipped. Native tests call async bindings directly, including the HTTP state contract and live reload, without subprocesses.
• Rustfmt, Ruff 0.5.2, Black 23.1.0, and diff whitespace checks passed.
• Changed files have CODEOWNERS coverage.