docs(mem): pin growth postures and outbox delivered-or-reclaimable semantic

Review 004 items 10+11: the two recorded mem growth postures
(subscribe backlog drain unbounded on the consumer side; O(live-rows)
claim scan with full-row clones) and the outbox's 'processed means
delivered-or-reclaimable' semantic, pinned in engine-mem.md. The
debug_assert arm was declined: the dropped ack/retry booleans also
refuse benignly on legitimate in-closure handle ops, so it cannot
separate a visibility lapse from a deliberate first-party outcome.

tasks/mem-doc-growth-outbox-notes.md

@@ -1,7 +1,7 @@
 ---
 id: mem-doc-growth-outbox-notes
 name: Pin mem's growth postures and outbox semantics in the spec (review 004 items 10+11)
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: trivial
@@ -60,8 +60,38 @@ Doc-only plus at most one `debug_assert!` — no behavioral change.
 
 ## Notes
 
-> To be filled by implementation agent
+- Neither `engine-mem.md` growth nor outbox section existed — both
+  were added (the review's "carry into the growth section" assumed a
+  touch that creates them). Growth postures placed after the
+  asymmetry-classification table; the outbox semantic as a
+  `### Outbox semantics` subsection under "Mapping the contract"
+  (the table's scheduler/outbox row pointed there).
+- The `debug_assert!` was **not** added (implementer's call, either
+  was offered): a debug assertion that the dropped ack/retry
+  booleans landed would also fire on legitimate in-closure handle
+  ops — a delivery closure that calls `ack`/`retry`/`fail` on the
+  boxed `JobHandle` (all available per ADR-019) makes the outer
+  `ack_row`/`retry_row` refuse benignly, so the assertion would
+  false-positive on valid flows and cannot distinguish a visibility
+  lapse from a deliberate first-party outcome without re-reading the
+  row's state (noise > value, debug-only). The doc sentence carries
+  the semantic; it also names the in-closure-op case the assertion
+  could not have separated.
 
 ## Summary
 
-> To be filled on completion
+Doc-only change, no behavioral code. `docs/architecture/engine-mem.md`
+gained: a `### Outbox semantics` subsection pinning "processed means
+delivered-or-reclaimable" for the no-heartbeat outbox (`run_once`
+returns `true` even when the ack/retry application refuses — at-least-once
+reclaim is the recovery), and a `## Growth postures (review 004 items
+10–11)` section with the two recorded postures anchored to
+`stream.rs:314-331` (subscribe drains the full backlog into pending
+before the first yield; pending grows at publish rate for a stalled
+consumer) and `queue.rs:122-181` (O(total-live-rows) claim scan with
+full-row clones — blessed ephemeral-tier economics). Frontmatter
+`last_updated` refreshed. Verified: `RUSTDOCFLAGS="-D warnings" cargo
+doc --no-deps` clean; `cargo test -p alkstore-mem` green (102 unit +
+26 suite + 1 doctest — outbox tests exercise the pinned `run_once`
+arms); `cargo clippy -p alkstore-mem --all-targets -- -D warnings`
+and `cargo fmt --check` clean.