feat(decisioning): add full stack registry factory

[sangilish]

Summary

Refs #697.

This adds PgBuyerAgentRegistry.with_full_stack() as the one-call factory for the documented production wrapper order: Caching(RateLimited(Auditing(pg))).

What changed

• Added with_full_stack() to PgBuyerAgentRegistry.
• Builds the canonical cache -> rate-limit -> audit stack while returning the outer CachingBuyerAgentRegistry.
• Threads audit_sink and sink_timeout_seconds through all three layers.
• Shares time_source between cache and rate limiter for deterministic tests.
• Wires Pg mutations through this registry instance to clear the returned cache.
• Documents the fixed layer order and points adopters with non-default ordering needs to manual composition.

Testing

• uv run ruff check src/adcp/decisioning/pg/buyer_agent_registry.py tests/conformance/decisioning/test_pg_buyer_agent_registry.py
• uv run mypy src/adcp/decisioning/pg/buyer_agent_registry.py
• ADCP_PG_TEST_URL=postgresql://postgres:[email protected]:55432/postgres uv run python -m pytest tests/conformance/decisioning/test_pg_buyer_agent_registry.py -q
• uv run python -m pytest tests/test_buyer_agent_registry_cache.py -q

[sangilish]

This is ready for review now. CI is green, and I kept the change scoped to #697: the documented Caching(RateLimited(Auditing(pg))) factory with mutation invalidation and audit/rate-limit coverage.

[bokelley]

Thanks for the heads-up — noted. The PR is queued for human review; a maintainer will pick it up from here.

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