fix: short-circuit identical compress failures (#156)

[ranxianglei]

Problem (#156)

Field logs from a 2B model (Codex + Responses, v0.1.27): the model retried the same failed compress range with the same validation error (Summary too short (28 chars…) ×6) for all 10 loop rounds. Every retry was a real upstream call — up to 10 wasted requests + tokens per session, with zero chance of the model self-correcting since it re-sent byte-identical arguments.

Fix

1. Identical-failure short-circuit (src/loop/core.ts)

compressFailureSignature() fingerprints each round's failed compress calls: requested refs (startRef..endRef, from parseCompressInput) + the failure text with volatile digits collapsed (so 28 chars and 31 chars count as the same failure class). After two consecutive rounds with an identical signature, runCompressLoop completes gracefully (finishReason: "length", same as the round-limit path) instead of re-requesting a third time.

• Distinct failures (model varies its range or error class) keep the loop alive — no premature stop.
• Any non-failing round resets the signature, so success → repeat-failure sequences still get their two chances.
• Logs [acp-loop] compress loop short-circuited (identical failure twice): … for observability.

2. Actionable 0-ranges failure text (src/stream.ts)

no valid ranges parsed now tells the model how to recover: each range needs string startId/endId refs plus a summary, and acp_status lists currently valid refs.

Test adaptation

tests/fix-stream.test.ts client-abort control arm replayed a byte-identical failing round, which the new short-circuit now (correctly) stops after 2 rounds. The fetch mock now varies the failing range per round so the control still measures what it was built to measure: without an abort signal, the loop runs to the round limit.

Tests

• New tests/compress-shortcircuit.test.ts (3): identical failure twice → exactly one re-request; distinct failures → loop continues; 0-ranges failure carries acp_status guidance.
• Full suite: 515/515 pass; tsc --noEmit clean.

Fixes #156 (items ① and ②; ③ shipped in v0.1.44 via compress.prompts; ④ tag-echo stripping tracked separately).