compaction: the pure core — cut walk, marker, and the model-facing projection (round 14 slice 1)

- core/compaction.ts: compactionCutIndex (backwards ~20k-token walk, NEVER
  separates a tool result from its call — the cut extends to the call's
  assistant message; a pending pose always survives), estimateTokens
  (chars/4, pi's method), newCompactionMarker (header + summary; details
  carry firstKeptMessageId/tokensBefore/model/at), serializeForSummary
  (labeled text, tool results truncated ~2k), planTreeToText (verbatim
  tree with transcript-derived statuses), buildCompactionPrompt (the four
  ruled sections: learner understanding + evolution, teaching arc, plan
  tree verbatim, current moment)
- toLlmMessages honors the cut: the ACTIVE (newest) marker renders ONCE
  as the head; everything before firstKeptMessageId drops; corrupt cut
  degrades honestly (everything renders, never throw)
- constants: KEEP_RECENT 20k, RESERVE 16k, DEFAULT_THRESHOLD 150k
- 16 test pins

src/core/index.ts

@@ -40,6 +40,7 @@ export * from "./math-delims.ts";
 // and the input-events contract — all
 // pure logic, browser-safe.
 export * from "./native-messages.ts";
+export * from "./compaction.ts";
 export * from "./input-events.ts";
 // Wave D (owner-approved 2026-09-26): the client view model — the grouping
 // canon (one continuous assistant document, step separators) ported from