fix(sim): gcEvents drops events still referenced by modifiers/factions/subjects

[csheldrick]

Summary

gcEvents() in src/sim/Events.ts swept state.events using a "referenced" set built only from eventLog, per-empire history, per-system recent events, and dynasty chronicle. Three other live pointers into state.events were never counted:

• RelationModifier.sourceEventId
• Faction.historicalEventIds
• SubjectRelation.historicalEventIds

The GC sweep runs every 128 events created, but a grievance modifier stays active for up to 3600 ticks — far longer than 128 events typically takes to elapse in a running galaxy. So the modifier's source event was almost always purged long before the modifier itself expired, silently corrupting:

• summarizeGrievances()'s oldestAgeTicks (a balance-tuning read-out surfaced in the headless report and in-app grievance summary), and
• the "from [tick] Title" annotation on grievances in the exported report (buildReport() in App.tsx), which just silently disappears once the source event ages out.

Confirmed empirically: at the same seed/tick count, oldestAgeTicks at tick 3000 went from 304 (before fix) to 637 (after fix) — matching the predicted corruption.

Changes

• Extend gcEvents()'s referenced-set collection to include RelationModifier.sourceEventId (via each empire's relationship modifiers), Faction.historicalEventIds, and SubjectRelation.historicalEventIds, before sweeping unreferenced events.
• Add src/sim/Events.test.ts covering all three previously-missed cases, plus a baseline test that true orphans are still collected.
• Record a Noet constraint capturing the durable rule this bug uncovered: any new Id reference into state.events must be added to gcEvents()'s referenced set.

Test plan

• npm run typecheck / npm run build
• npm run lint
• npm run test:unit (9/9 passing, including 4 new tests in Events.test.ts)
• Verified all 3 targeted regression tests fail against the pre-fix code and pass against the fix
• npm run report (headless health check, 1000/3000-tick milestones) — passes, and oldestAgeTicks now reflects the fix
• Determinism replay check passes (npm run report --milestones 500, byte-identical replay)

---

Generated by Claude Code