docs(tasks): decompose wave 2 — mem + local engines, facade, in-process GC, first suite columns

Nine tasks against the wave-1 contract surface as reviewed (no
amendments from the gate):

- engine-mem / engine-local (parallel, dependency-free): the two
  engines; local owns the stage-hygiene exact-count tests and the
  Drop-cancellation story the wave-1 review recorded
- store-construction: the Store type, constructor modes, size-threshold
  dispatch; binds the conformance factory's Store associated type
- store-read-paths ∥ gc-protocol: read ops with the ADR-012 §6.1
  fall-through rule; the ADR-005 in-process protocol (pins, liveness,
  delete windows, abort-as-data, GcRefuse with the full protection set)
- store-put-paths: the two put paths behind one signature, ADR-003's
  three numbered rules, pin-before-publish, Batch, kv-only rejection
- suite-column-mem → suite-column-local: the reference column, then the
  dual-tier dispatch-coverage shape mirroring it
- review-wave-2: the gate (facade/dispatch/GC vs ADR-003/005/012)

Verification: dependency graph validated (all ids resolve, no cycles,
topological order clean); docs-only change.

Refs: docs/plans/implementation.md §Wave 2

tasks/suite-column-local.md

@@ -0,0 +1,80 @@
+---
+id: suite-column-local
+name: Conformance suite — the local column and the dispatch-coverage shape
+status: pending
+depends_on: [suite-column-mem]
+scope: moderate
+risk: medium
+impact: phase
+level: implementation
+tags: [wave-2, suite, local, dispatch]
+---
+
+## Description
+
+Grow the **local column** — the suite's first dual-tier column, run
+against the `kv = mem` + `large = local` factory: the
+**dispatch-coverage test shape** ADR-012 §6.3 names (both tiers
+load-bearing). Rows mirror the mem column's set (INV-7: every engine's
+rows mirror the reference column), extended with the dual-tier and
+local-specific properties.
+
+- **The factory**: fresh, isolated temp-root dual-tier store per call
+  (`kv = mem` + `large = local`), idempotent teardown (temp dir
+  removal); never skips.
+- **Mirrored rows** (the mem column's set, against the dual-tier
+  store):
+  - **INV-1 round-trips**: both tiers, both put paths — the content
+    matrix crosses the threshold so both tiers' bodies are exercised
+    (sub-threshold kv rows; over-threshold local stage-commit rows).
+  - **INV-3 stage hygiene**: exact counts **on disk** — zero stage
+    files after every failure form, exactly one committed entry per
+    success, over both put paths (the real stage form, not the
+    degenerate kv stage).
+  - **INV-4 sweep safety**: exact-count sweep outcomes over the
+    dual-tier pool (candidates from both tiers; abort shapes; the
+    sweep-vs-pinned-put race).
+  - **INV-2 dedup, INV-5 list-through-sweep, INV-6 stat** mirrored.
+- **Dispatch rows** (the coverage shape's reason to exist):
+  - Routing is a pure function of length: same content ⇒ same tier
+    across mixed put paths and re-puts; no content ever migrates.
+  - **Fall-through** (ADR-012 §6.1): a kv-tier miss queries the large
+    tier — `get`/`stat`/`has`/`read_range` each served from the large
+    tier when the kv tier misses; both-tier miss shapes per op.
+  - Threshold boundary values (at/below/above the default; a
+    constructor-tuned threshold re-routes deterministically).
+- **Local-specific rows**: pread range reads over committed files
+  (mid-file slices, EOF clamping); virgin-store no-ops on a fresh root
+  (empty `list`, absent reads); `size` as a stat (never a content
+  load).
+- Every row ADR-stamped per the convention, tracing to the backlog.
+
+## Acceptance Criteria
+
+- [ ] The dual-tier `kv = mem` + `large = local` factory green with
+      isolation + idempotent teardown
+- [ ] The local column green: mem's row set mirrored (INV-1/2/3/4/5/6)
+      plus dispatch rows (pure-function routing, fall-through per op,
+      threshold boundaries) and local-specific rows (pread ranges,
+      virgin-store no-ops, stat-as-stat)
+- [ ] Stage-hygiene exact counts asserted on disk (the real stage form)
+- [ ] Every row ADR-stamped and traceable to the backlog
+- [ ] `cargo test` green (local is ungated — the column runs in the
+      default build); clippy/fmt gates green
+
+## References
+
+- tests/conformance/README.md + backlog.rs (the conventions; INV first-column plan)
+- docs/architecture/backends-and-dispatch.md (§Constructor modes — the dispatch-coverage shape; §Size-threshold dispatch)
+- docs/architecture/decisions/012-pre-decomposition-consistency-rulings.md §6.1/§6.3
+- docs/architecture/decisions/003-backend-contract-and-dispatch.md (§Dispatch policy)
+- docs/architecture/store-api.md (§Invariants)
+- docs/research/poc-largeblob-findings.md (findings A2/A5)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion