[wusijian007]

Lay the foundation for the v4 five-layer compaction pipeline: a pre-flight cascade whose budget trigger hangs on a usage anchor instead of a whole- transcript chars/4 guess. Structural refactor with zero behavior change — the spine carries the single existing stage (semantic when a summarizer is injected, else tiered); the real layers L1-L5 land in M4.1-M4.5.

Usage anchor (estimateAnchoredTokens): the last API usage gives the EXACT server-side prompt-token count of the strict-extension prefix that was already sent (system + tools + the first N messages); only the messages appended since are estimated (chars/4). Error stays small because the bulk is exact and only the recent delta is guessed (<5%). The loop re-anchors after every model turn from that turn's usage + the sent message count, and INVALIDATES the anchor on a compaction (the prefix was rewritten). Falls back to the full heuristic when there is no anchor or the transcript is shorter than the anchored prefix.

Cascade spine (runCompactionPipeline): runs CompactionStages cheapest-first and short-circuits as soon as the transcript is under target — same shape as the M3.5 done-gate chain. The proactive turn-boundary block now computes the anchored estimate, and on crossing the soft limit runs the pipeline toward the target ratio.

• core: estimateAnchoredTokens + UsageAnchor, CompactionStage + runCompactionPipeline (context.ts); query loop re-anchors per turn, triggers off the anchor, invalidates on compaction.
• eval: semantic-compaction's scripted turn-0 usage raised to match its seeded whale (the anchor now trusts server token counts, so fixture usage must be consistent with seeded content) — fingerprint in 14350 -> 15850, rest stable.
• tests: 6 new (anchor exact-prefix + 2 fallbacks; pipeline short-circuit + no-op; anchor-driven trigger suppresses a compaction chars/4 would have fired).
• docs: CLAUDE.md query-loop step 7 rewritten around the cascade + anchor; v4 roadmap M4.0 marked delivered.

226 tests pass (+6); all 10 eval tasks green.