Add opt-in event_entity KG builder, event_join partitioner and event extraction profiles

[Leon-Algo]

Summary

• Opt-in event pipeline: kg_method: event_entity builds an EVENT star graph via a profile-driven extractor; partition.method: event_join groups same-anchor events. Default light_rag + ECE route is untouched.
• Event profiles: graphgen/templates/event_profile_loader.py validates manifest+prompt fail-closed with SHA-256 lineage. Ships only generic_event.v1 as an example; no default profile — event_entity mode without an explicit event_profile errors instead of falling back to a domain policy.
• External profile paths preserved end-to-end: BuildKGService validates at init but passes the original path reference through to the builder (fixes the second-load event_profile_not_found bug flagged in Add versioned prompt profiles and event-entity generation pipeline #192).
• Support citations fixed: <support> edge citations accept the rendered -- separator and both endpoint orders, canonicalize to stored orientation, and closure validation accepts only edges that actually exist in the partition (fixes the always-stripped citations bug flagged in Add versioned prompt profiles and event-entity generation pipeline #192).
• Membership edges self-describing: extract() sets event_member=True at creation and deterministic merge persists the participant role (fixes direct-builder misuse and role loss flagged in Add versioned prompt profiles and event-entity generation pipeline #192).
• Robust partitioning: a missing-anchor event is skipped (consistent with extraction's drop-and-count) instead of raising and killing the whole partition stage.
• No PMS profiles/gates/docs, no import-time file I/O, no fork identity.

Third of three PRs split from #192 per review. Depends on #193 (partitioner test fixes; the ECE integration tests don't collect on main) and #194 (profile/prefill/LLM mechanisms this builds on).

Validation

• pytest tests/unit_tests → 13 passed: disk-profile service→builder propagation, citations both orders + strict closure, anchor skip, merge role persistence, direct extraction marker, loader validation.
• Default-route check: PR A's synced ECE tests run green against this branch's production code.
• pylint on all touched modules → 10.00/10.

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