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/engine-mem.md

@@ -0,0 +1,76 @@
+---
+id: engine-mem
+name: mem kv engine — the contract-reference engine
+status: pending
+depends_on: []
+scope: narrow
+risk: low
+impact: component
+level: implementation
+tags: [wave-2, engines, mem]
+---
+
+## Description
+
+Implement the `mem` kv-tier engine (feature `mem`; ADR-011 §2): a
+`BTreeMap`-shaped ephemeral implementation of the kv tier's contract —
+the first real engine, and the **contract-reference engine**: every
+other engine's conformance rows mirror its column (store-api.md
+invariant 7), so its bodies are the shapes the suite holds the rest to.
+
+- **Backend impl** (`src/mem.rs`, behind `#[cfg(feature = "mem")]`)
+  against the wave-1 trait (`src/backend.rs`): the seven methods plus
+  `put_staged`, opaque byte keys/values, `name() -> "mem"`.
+  - `put` — CAS first-writer-wins (`entry().or_insert()`); a re-put of
+    an existing key is a no-op, never an overwrite.
+  - `get` — `None` on absence (never `Error::Missing`); the cursor body
+    is the **materialized family** (the whole value; `len()` from the
+    stored value).
+  - `put_staged` — the **degenerate in-memory stage** (ADR-010 §1: kv
+    bodies stage in memory and commit through the whole-value row
+    write); `append` extends the buffer, `commit` publishes via the
+    same CAS clause and self-discards on failure, `discard` drops the
+    buffer with zero residue.
+  - `delete` — `Err(Error::Missing)` when absent; `list` — complete and
+    well-formed (the map's keys); `size` — trivially metadata-only (the
+    stored value's length, no content fetch semantics to violate).
+  - Virgin-store reads are no-ops (an empty map is the virgin store).
+- **Locking posture**: `parking_lot` for the short-held map lock
+  (AGENTS convention 3; `unwrap_or_else(|e| e.into_inner())` on
+  poisoning). No blocking I/O exists here.
+- **Unit tests** in-module: the method contract (virgin-store no-ops,
+  CAS re-put, Missing discipline, size exactness), the staged-put happy
+  path and both hygiene forms (discard → zero residue; commit failure →
+  self-discard) — mirroring the wave-1 test-double's assertions, now
+  against the real engine.
+
+The conformance column itself is `suite-column-mem` (separate task);
+this task ships the engine and its in-module unit tests only.
+
+## Acceptance Criteria
+
+- [ ] `src/mem.rs` implements `Backend` completely (seven methods +
+      `put_staged`) behind feature `mem`, with the materialized cursor
+      body and the degenerate staging body
+- [ ] CAS first-writer-wins on both put forms; re-put never overwrites
+- [ ] Stage hygiene holds in both forms (discard → zero residue;
+      failed commit → self-discard), unit-tested
+- [ ] Virgin-store reads are no-ops; `delete`/absent-key discipline per
+      the trait contract; `size` metadata-only
+- [ ] `cargo test --features mem` green; `cargo clippy --all-features
+      --all-targets -- -D warnings` and `cargo fmt --check` green
+
+## References
+
+- docs/architecture/backends-and-dispatch.md (§kv tier — mem engine)
+- docs/architecture/decisions/011-vocabulary-tiers-engines-and-fleet-state.md §2
+- docs/architecture/decisions/010-backend-io-seams-and-fleet-state-home.md §1
+- src/backend.rs (the trait + seam contracts this implements)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion