Improve Code Health of hotspot source files

[woksin]

Changed

• The event sequence, the client and the reducers are restructured into smaller, focused units for maintainability, with no change in behaviour or public API (Improve Code Health of the Chronicle.TypeScript hotspot source files #244)

[woksin]

Reviewer notes

Behaviour-preserving refactor of CodeScene hotspot source files (CodeScene project 85913). Tests, specs, docs and config are untouched; the existing suites are the safety net.

• Code Health (baseline -> now): EventSequence.ts 6.56 -> 9.38; ChronicleClient.ts 7.97 -> 10; Reducers.ts 7.74 -> 9.68
• Local Tier 1 (mirrors CI) passed on the final tree: yarn ci (3319 tests); yarn test:integration not run locally (needs kernel container)
• Reviewed by a cross-provider review (Claude Opus + GPT) with adversarial verification; behaviour changes found during iteration (evaluation order, async-context flow, resource lifetimes, visibility) were fixed before this PR.
• Integration jobs run in CI only (Docker is not available locally).