feat(chunk-store): ADR 0072 phase 1 — write-through floor (#548)

[nikhilunni]

Phase 1 of epic #548 (GCS-free resume). ADR 0072 (Proposed). Branches off main (has the #528/#529 deps).

The mechanical write-through floor — moves 4+5 of the ADR. Backend-agnostic, no wire change.

Changes

• Move 4 — close the exists() write-through hole (engram-chunk-store/src/store.rs): put_chunk's idempotency short-circuit now runs the local write-through (warm_local) before returning. A chunk that is remotely present but locally evicted is re-warmed by a put, so the producing host stops re-fetching its own uploads from GCS on the next resume. (Closes the 583+55 write_local failed / write-through … failed re-miss class for the exists-arm.)
• Move 5 — drop the per-dirty-chunk GCS HEAD on flush (store.rs + both flush call sites): new put_chunk_unchecked(body) — unconditional PUT + unconditional local warm, no exists(). The NBD disk flush (disk_daemon/backend.rs) and teleport memory catch-up (pooled_backend.rs) switch to it: their input is freshly re-chunked, new by construction, so the per-chunk HEAD was O(dirty) round-trips/flush that always missed. put_chunk keeps its dedup HEAD for capture/enable-scale uploads (there it saves re-uploading tens of GiB on a re-bake) — the fix is a second entry point, not a behavior change.
• Metric engram_chunk_put_total{mode=checked|unchecked, outcome}. The divergence_source{tier} counter lands in phase 3, where local|peer|gcs are all real (emitted at the resume miss-chain) — not redundantly bolted onto the generic cache hit/miss path now.

Tests

Counting-mock BlobStorage asserts: exists()-arm warms a locally-evicted chunk (move 4); put_chunk_unchecked issues zero HEADs + uploads + warms (move 5, acceptance #2); a 16-chunk flush issues zero HEADs; put_chunk still dedups on a second identical put (capture-path regression guard). All green; cargo check -p engram-host-agent clean; clippy + fmt clean.

Notes

• ADR numbered 0072 (above main's 0067-0069 and my in-flight epic branches' 0070/0071) to avoid collision.
• Phases 2 (authoritative affinity + retire local_snapshots, wire+schema), 3 (peer-first fill, rides Substrate single-writer discipline: one chunk-cache owner per host (interim) + engram-substrated ADR (design only) #547), 4 (evict-time prestage + rung-3 parked-local), 5 (flip to Accepted) follow as stacked PRs.

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	⏩ skipped	⏩ skipped	❌ failed (5)	Jul 6, 2026, 1:31 PM