feat(plugins): backend-engine-source-cache plugin (Phase 5 #50)

[charlie83Gs]

Summary

• Third Phase-5 provider plugin: plugins/backend-engine-source-cache/
• Contributes public-graph SourceCacheProvider matching GraphTypeComposition.source_cache on the default graph type
• Wraps existing PublicGraphBridge read path exposed on WorkerGraphEngine (lookup_public_cache / import_from_public) — zero behaviour change

What changes

• PublicGraphSourceCacheProvider implements the SourceCacheProvider ABC. Constructed once per worker; initialize(services) stashes the CoreServices container for idempotent re-init.
• Per-call engine resolution: each lookup / import_into_target resolves a freshly-bound WorkerGraphEngine for target_graph_id via services.graph_engine(target_graph_id). The engine already carries the right bridge (or none — both calls no-op per contract).
• Engine's raw ImportResult gets packed into the typed SourceCacheImportResult at the boundary so observability dashboards line up with the ABC.
• Degraded / malformed results (stale snapshot, missing raw_source_id) resolve to None + log — matches ABC's "cache miss is a degraded result, not an error" contract.
• Fail-fast on lookup before initialize: silent None would mask wiring bug as indistinguishable from upstream down.
• ctx.source_graph_id advisory for now — engine binds to default-graph upstream at construction (today's WorkerState.make_worker_engine hardcodes). Custom compositions that want a different upstream need bridge itself to accept source id — follow-up.
• Registered in kt_config.plugin.load_default_plugins. No worker wiring this PR — mirrors definition / sync plugin-then-wire sequencing.

Test plan

• uv run --project plugins/backend-engine-source-cache pytest -x -v (12 contract tests green — plugin identity, is_enabled, contribution id, registry round-trip, initialize idempotency, lookup + import delegation, no-bridge no-ops, degraded engine results, not-initialized fail-fast)
• uv run --project libs/kt-config pytest -x -v (236 green)
• uv run --frozen ruff format --check . / ruff check .
• Pre-push hook ran full unit test suite — all green

🤖 Generated with Claude Code

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