Embeddings: verify darwin exit-6 crash clears with next release; add backoff/breaker for model-download retry loop

[h4yfans]

Summary

Follow-up bundle for the embeddings utility. The biggest crash group of the triage (macOS exit 6) should already be fixed by merged-but-unreleased PRs — this issue tracks release verification plus one real gap: the model-download retry loop has no backoff/breaker.

Evidence (Loki, prod, Jul 19–22)

1. Utility:crashed:Embeddings child_process_gone exit 6 — ×42 / 8 installs, all darwin, all v2026.719.2. Fixes fix(updater): stop embeddings utility process on quit (#805) #820 (stop utility process on quit) and fix(vault): defer embeddings out of the index pass so vault-open never hangs (#803) #821 (defer during index + loadFailed breaker) are merged on main, but git tag --contains 582db0cb3 is empty → they are in no shipped release.
2. One win32 install, 10-minute window (07-21 10:47→10:57): Embeddings:Worker — Failed to load embedding model {message:'fetch failed'} ×48, each mirrored by an Embeddings stderr line (96 log lines total). Model download failing (offline/proxy/blocked CDN) → tight retry loop, no backoff.
3. Noise: Embeddings stderr — Unable to determine content-length from response headers… ×10 / 9 installs (informational, logged as error).

Actions

• Cut the next release (ships fix(updater): stop embeddings utility process on quit (#805) #820/fix(vault): defer embeddings out of the index pass so vault-open never hangs (#803) #821).
• After release: re-query Loki, confirm the exit-6 crash group disappears for the new version ({app="desktop",kind="error"} |= "Utility:crashed:Embeddings" filtered by app_version).
• Verify fix(vault): defer embeddings out of the index pass so vault-open never hangs (#803) #821's loadFailed breaker actually covers repeated download fetch failed (not just model-load failures). If not, add exponential backoff + circuit breaker (stop after N failures until next app start or network-online event).
• Route the content-length stderr line to debug level.

Code pointers

• apps/desktop/src/main/lib/embeddings.ts (utility lifecycle, stderr forwarding)
• Related: [Bug] Opening a vault hangs indefinitely (>5 min) — embeddings run inline before the vault is marked open #803 (vault-open hang), [Bug] Windows update refuses to install ("MemryNote cannot be closed") — embeddings utility process not terminated on quit #805 (Windows update blocked by phantom process)

Is the log data sufficient?

Yes. Crash + retry-loop patterns are fully characterized; this is release + verification + a targeted backoff fix.

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Part of #836 (prod log triage epic).