feat(event): compose TDT and CHRONOS as one versioned workflow

[seonghobae]

Summary

Remaining #170 composition slice only: one versioned unified TDT/CHRONOS workflow inside existing event_core (no new crate).

• Admits already-extracted artifacts in sequence: segmentation → span-grounded mentions → links → first-story → tracks → schema slots → forecasts (Allan, 2002; Li et al., 2021; Anagnostopoulos, Batsakis, & Petrakis, 2013).
• Composition refuses instance/transition casting; admit_state_transition(composition.evidence_layer()) is not PromotedTransition.
• Later revised-document mentions append without rewriting earlier mention_id / source_span / track identities.
• Known-truth contract covers exact-extent mention P/R, WindowDiff/Pk, link/track pair P/R, first-story miss/FA, schema-slot P/R, and forecast Brier via existing helpers.

Not in this slice: interval consistency, persistence, JSON/JSON-LD/GraphML exports (later #170).
Do not touch / do not convert: open psychometric PRs #267 and #268.

Changelog note (MCP size blocker)

Intended Unreleased bullet (prepend):
event_core EventMention composition is now a versioned TDT/CHRONOS workflow (segmentation → span-grounded mentions → links → first-story → tracks → schema slots → forecasts) and still cannot promote instances; interval consistency, persistence, and exports remain later #170 work.

Full CHANGELOG.md rewrite via GitHub MCP was blocked by ~218KB payload size; please land that one-line prepend in a follow-up commit before merge if CI/docs require it.

Merge gate

Do not merge without independent APPROVE and green exact-head product CI.

Closes nothing by itself; advances #170 composition only.

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