Addresses PR #3412 review (roboomp blocking + codex P2 + Copilot nits):
- The overflow/incomplete retry no longer reuses the COMPACTION_RECOVERY_BAND
hysteresis. Reusing it required residual context below 0.8×threshold, which
turned a recoverable overflow (e.g. 150k on a 200k window, under the ~170k
threshold) into a manual dead-end. Add #compactionCreatedRetryFit, which
measures the rebuilt prompt against the usable fit budget
(contextWindow - effectiveReserveTokens) and is evaluated AFTER the failed
assistant is dropped, so the just-failed turn is excluded.
- The threshold auto-continue keeps the stricter recovery-band check
(#compactionCreatedHeadroom) — that path is the snapcompact thrash guard.
- Split the no-progress warning so each path warns on its own signal.
- Alias errorIsFromBeforeCompaction as assistantPredatesCompaction in the
threshold-usage path for clarity; de-duplicate the rationale comment.
Adds two regression tests (recoverable overflow retries; non-fitting overflow
pauses + warns), mutation-verified against the band-vs-fit split.
(cherry picked from commit f8cf45b2740f04d8776a84521ba563b779d65b25)
