release: N-1 growth postures consciously accepted (all three engines)

The wave-8 N-1 deferred item — re-read the three unconditional-growth
paths against the implemented engines and land the decision of record:
all three accepted for v1, per engine, with no engine-side hygiene.

- deployment.md: new 'Growth postures (consciously accepted)' section —
  per-engine table (dead letters sweep-on-call, SQLite-only notify
  at-attach cap, durable-log stream posture; mem process-lifetime-
  bounded by design) + the rationale and the deployment-matrix
  fold-in positioning.
- engine-sqlite.md: 'Notifications-table hygiene' — attach is the only
  prune site (D-16, oldest-first 10k); idle-but-listening processes
  never prune; the cadence caveat deployers should size for.
- tasks/release-n1-growth-posture.md: completed; Notes/Summary filled.

tasks/release-n1-growth-posture.md

@@ -1,7 +1,7 @@
 ---
 id: release-n1-growth-posture
 name: N-1 growth-posture review — conscious acceptance (or hygiene)
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: low
@@ -50,16 +50,16 @@ design; say so in the record), and land the acceptance:
 
 ## Acceptance Criteria
 
-- [ ] The three growth paths re-read as implemented (SQLite + pg +
+- [x] The three growth paths re-read as implemented (SQLite + pg +
       mem); the review's characterization confirmed or corrected in
       the record
-- [ ] The conscious-acceptance decision recorded per engine (or a
+- [x] The conscious-acceptance decision recorded per engine (or a
       user-approved hygiene scope change, which would be a follow-up
       task)
-- [ ] The notify prune-cadence note landed in the SQLite engine's docs
-- [ ] The acceptance substance positioned for the deployment-matrix
+- [x] The notify prune-cadence note landed in the SQLite engine's docs
+- [x] The acceptance substance positioned for the deployment-matrix
       final pass to fold in
-- [ ] Workspace gates green (docs-only expected)
+- [x] Workspace gates green (docs-only expected)
 
 ## References
 
@@ -72,8 +72,56 @@ design; say so in the record), and land the acceptance:
 
 ## Notes
 
-> To be filled by implementation agent
+- **Decision: conscious acceptance, all three paths, all three
+  engines.** No engine-side hygiene scope change proposed or wanted.
+- **The review's characterization confirmed, with one sharpening per
+  path** (verified against code, not just docs):
+  - *Dead letters*: the characterization held, plus a sharpening the
+    review didn't state — retention enforcement has exactly **one
+    trigger**: a caller-driven `sweep_expired()` (`Queue` trait
+    method). Engines run no background retention sweeper (the pg
+    suite's comment says it outright: the sweep is "the only sweeper
+    dead rows ever have"). So growth is opted-in twice: retention
+    `None` by default, and no automatic cadence when it's set.
+  - *Notifications*: held — `NOTIFY_ATTACH_MAX_ROWS` = 10,000,
+    oldest-first, runs only at connection open (the substrate
+    bootstrap, `schema.rs`); the D-16 cap triggers **only** on a
+    fresh attach. Confirmed SQLite-only: pg has **no notifications
+    table** (`pg_notify` is transient, ≤ 8000 bytes; pg schema has no
+    such table). Upstream's `prune_notifications*` helpers were not
+    carried (ADR-008 §4's disposition), so attach is the only prune
+    site.
+  - *Stream*: held — no engine-side trim beyond the consumer-invoked
+    `trim_to` (ADR-015 §5's posture: default unbounded, documented
+    squarely).
+- **The mem engine's postures (new since the review)**: its
+  notify/stream/dead growth is process-lifetime-bounded by design —
+  notify stores nothing (commit-boundary delivery to open receivers;
+  per-subscriber feeds are unbounded mpsc, ending at engine drop),
+  the stream log and dead table are in-memory state erased at process
+  exit. Accepted structurally rather than via caps; recorded in the
+  deployment.md acceptance.
+- The acceptance record landed in `deployment.md` §"Growth postures
+  (consciously accepted)" with an explicit "this record is the input
+  the deployment-matrix final pass folds in" positioning; the notify
+  prune-cadence note landed in `engine-sqlite.md` §"Notifications-
+  table hygiene" (anchored heading — deployment.md links to it). No
+  internal task pointers in the release-facing text; the matrix pass
+  can fold the substance directly.
 
 ## Summary
 
-> To be filled on completion
+Re-read the three growth paths against the implemented engines
+(`alkstore-sqlite/src/substrate/schema.rs` (D-16 at-attach prune),
+`queue_ops.rs` (`sweep_dead_retention` inside the savepoint guard,
+sweep-on-call only), `alkstore-postgres/src/schema.rs` (no
+notifications table), `queue.rs`/`stream.rs` sweep/trim shapes, and
+`alkstore-mem/src/{notify,wakes,queue,stream}.rs`). Decision of record
+(**all three accepted for v1, per engine**, no hygiene scope change):
+landed in `docs/architecture/deployment.md` §"Growth postures
+(consciously accepted)" (per-engine table + rationale, frontmatter
+updated), and the notify prune-cadence note (attach is the only prune
+site; idle-but-listening processes never prune) landed in
+`docs/architecture/engine-sqlite.md` §"Notifications-table hygiene"
+(frontmatter updated). Docs-only change; workspace gates green
+(`cargo build`, `cargo test`, clippy `-D warnings`, fmt --check).