ADR-024 — the mem engine adopted as the family's third engine (wave 6; release readiness renumbered to wave 7)

Row-first gate: consumer-inventory.md gains an engine-tier section — the mem-engine need recorded operator-authority, dated 2026-10-10 (wasm32 sandboxing across the alk protocol family + downstream ffi/napi/python-adapter economics; rusqlite/tokio-postgres structurally out on wasm, so mem is the only triad member the sandbox can carry).

ADR-024 (decisions/024-mem-engine.md) pins the decision set: (1) separate `alkstore-mem` crate — discharges and amends ADR-001 §4 (superseded in part: full contract-v1 engine, not an "implementation convenience"; annotation on ADR-001 §4 + Status, class-1 window still open); (2) honest-ephemeral posture — no durability, single-process, ephemeral by design, never fleet-valid (ADR-010 §3's shared-pool predicate fails structurally), riding ADR-016's carriers unchanged (identity + docs + matrix); (3) full contract v1 tier with the asymmetry classification — `PayloadTooLarge` never produced (SQLite arm), no reconnection concept (reconnect-wake/watcher rows absent from mem's per-engine column), receiver close at engine drop, wake coalescing N/A; (4) `MemStore::new()` engine-native constructor — the one documented exception to the open-prose, core-contract.md Store concept amended; shared-instance (two opens → one engine) rejected with the ADR-016 §4-style re-entry gate (consumer-inventory row), register stays closed; (5) wasm32 — compile-clean for wasm32-unknown-unknown plus the suite's mem column green on host under a current-thread runtime flavor are wave-6 acceptance items; on-target suite execution scoped out with a named collapse condition (wasm-bindgen-test adapter crate) — a scope decision, not a pending-client hedge; (6) mem is not a core dev-dependency — core's doc examples stay mock-based, ADR-001 §4's doctest posture declines.

engine-mem.md (draft): the in-process mapping — guarded per-mechanism state (no pool, no schema bootstrap, no forwarder, no spawn_blocking seam), the tx overlay (staged *_tx effects merged atomically at commit; read-your-own-writes easy, drop-=-rollback trivial; wake-at-commit structural — the seam pg needed pg-fix-tx-wake for, mem pins from birth via the existing tx-commit-atomicity rows), ADR-012 §2 third owner of the curve/opts/@every arithmetic (three-way equivalence, ADR-022's StoreFactory reused verbatim — born pinned).

deployment.md: host-semantics row (single-process, ephemeral, never fleet-valid, wasm32-clean), mem connection-budget subsection (none — nothing to budget), durability-knobs row (the honest row is the absence), toolchain row (wasm32 subset + current-thread-clean, acceptance-cited). README/overview: engine-mem.md row, ADR-024 row, family table entry (alkstore-mem, no driver deps), current-state updated to the implementation phase with engine specs' stable statuses reflected.

implementation.md: wave table gains wave 6 (mem engine, depends waves 1 + 5) and renumbers release readiness to wave 7 (fuzzing decision — the one remaining decided-at-implementation deferral — rides wave 7); wave-6 section records the born-pinned posture, acceptance items, the one suite-harness pre-task (past_stamp_sleep blocking sleep → async sleep, review 003's properties.rs:1683 note), and the window riders (review 003 Finding 1: the pg transient-fault seam, MEDIUM, rides wave 6; Findings 2–3 follow their park into wave 7's pre-release); wave-6 review gate defined; Decided points updated (mem discharged, fuzzing renumbered); stale wave-6 references swept across review docs and the landed tasks' notes (release-readiness items renumbered or annotated).

Docs-only change; verified by cross-reference audit (all ADR/OQ/anchor references resolve, residual wave-6 mentions are correct under the new numbering or carry dated annotations).

docs/architecture/README.md

@@ -1,23 +1,29 @@
 ---
 status: draft
-last_updated: 2026-10-08 (ADR-023: fourth review round resolved — waves-1–2 general-review findings)
+last_updated: 2026-10-10 (ADR-024: the mem engine adopted as a third
+engine — engine-mem.md added, ADR table row, index updated for the
+implementation phase)
 ---
 
 # alkstore — Architecture
 
 Architecture documentation for the alkstore project: one reactive
 store interface (notify, streams, queues, locks, scheduler, outbox)
-over SQLite and Postgres, with each engine native underneath
-(see [overview.md](overview.md)).
+over SQLite, Postgres, and the in-process mem engine, with each
+engine native underneath (see [overview.md](overview.md)).
 
 ## Current State
 
-**Phase 1 (Architecture) — in progress.** Phase 0 is complete
-(`docs/research/phase-0.md`): both POCs ran and passed, the scope
-inventory is confirmed, and the crate split, drivers, and ownership
-postures are decided. This directory carries the architecture spec
-build-out over that evidence base; all spec documents are `draft`
-pending architecture review and OQ resolution.
+**Phase 1 (Implementation) — waves 1–5 implemented and reviewed.**
+Phase 0 is complete (`docs/research/phase-0.md`): both POCs ran and
+passed, the scope inventory is confirmed, and the crate split,
+drivers, and ownership postures are decided. All open questions are
+resolved (through ADR-023), and the engine specs flipped `stable` at
+the wave-5 review gate. Waves 1–5 (core, substrate fork, SQLite
+engine, Postgres engine, contract suite) are implemented and reviewed
+per `docs/plans/implementation.md`. The mem engine was adopted as a
+third engine by [ADR-024](decisions/024-mem-engine.md) (2026-10-10,
+wave 6 — release readiness renumbered to wave 7).
 
 ## Architecture Documents
 
@@ -25,8 +31,9 @@ pending architecture review and OQ resolution.
 |---|---|---|---|
 | [overview.md](overview.md) | draft | Crate family, feature surface, non-goals, evidence base | — |
 | [core-contract.md](core-contract.md) | draft | The unified trait surface, delivery guarantees, tx seam | OQ-10 (resolved) |
-| [engine-sqlite.md](engine-sqlite.md) | draft | SQLite engine: forked-substrate/rusqlite mapping | OQ-06 (resolved), OQ-12 (resolved), OQ-13 (resolved) |
-| [engine-postgres.md](engine-postgres.md) | draft | Postgres engine: tokio-postgres/LISTEN mapping | OQ-08 (resolved), OQ-12 (resolved), OQ-13 (resolved) |
+| [engine-sqlite.md](engine-sqlite.md) | stable | SQLite engine: forked-substrate/rusqlite mapping | OQ-06 (resolved), OQ-12 (resolved), OQ-13 (resolved) |
+| [engine-postgres.md](engine-postgres.md) | stable | Postgres engine: tokio-postgres/LISTEN mapping | OQ-08 (resolved), OQ-12 (resolved), OQ-13 (resolved) |
+| [engine-mem.md](engine-mem.md) | draft | Mem engine: in-process ephemeral mapping of the contract ([ADR-024](decisions/024-mem-engine.md)) | — |
 | [queues.md](queues.md) | draft | Queue/scheduler/outbox semantics depth (resolved: ADR-009/ADR-010) | OQ-05 (resolved), OQ-09 (resolved), OQ-06 (resolved) |
 | [deployment.md](deployment.md) | draft | Host semantics, connection budgets, knobs, matrix | OQ-08 (resolved) |
 | [open-questions.md](open-questions.md) | draft | OQ tracker (promoted from OQ-ST register) | — |
@@ -58,6 +65,7 @@ pending architecture review and OQ resolution.
 | [021](decisions/021-tx-reads-and-value-shape-fixes.md) | Third review round — tx-read methods on `TxHandle`, `Job.claimed_at`, `schedule()` queue-argument validation, drop = rollback, receiver close/error arms | Accepted |
 | [022](decisions/022-contract-suite-layout.md) | Contract-suite layout — shared internal suite crate, properties parameterized over a store factory (discharges ADR-017 §4.2's deferral) | Accepted |
 | [023](decisions/023-fourth-review-round.md) | Fourth review round — `encode_payload` typed (`Codec`), numeric-argument domains by kind (extents clamp empty, durations reject, boundaries total), plain-path SQLite open (URI flag dropped), watcher cadence 1 ms default + `SqliteOpts` knob | Accepted |
+| [024](decisions/024-mem-engine.md) | The mem engine — a third engine (`alkstore-mem`), full contract v1, honest-ephemeral posture (collapses + amends ADR-001 §4's deferral) | Accepted |
 
 ## Open Questions