docs(review): the wave-2 review gate — every gate check green

Read the wave-2 implementation against the pinned ADR text line by
line (ADR-003/005/006/010/011/012/013 §2, store-api.md): the three
numbered unknown-length rules each test-backed, the delete-window
ordering invariants (arbitrate-at-delete-time, pin-before-publish on
both put paths and both tiers, raced conversion ordering,
delete-then-recover), abort-as-data with all three causes, dispatch
purity + fall-through + constructor modes + kv-only's two put-time
rejection points, stage-hygiene exact counts on both engines (incl.
the Drop-cancellation story), both suite columns green/ADR-stamped/
traceable with the factory binding recorded, the dispatch-coverage
shape exercised, all eight named design points recorded and
consistent, and the staged-put key-at-commit amendment audited
accepted. No wave-3 amendments arise.

Inline fixes applied by the gate:
- facade.rs: stale Batch::add dead-code allow + stale task-pointer
  comment removed (put_in_batch's call sites landed)
- dispatch.rs: module-level allow(dead_code) removed (route/Tier now
  driven; clippy clean without it), scope note updated
- backend.rs: the allow kept but its scope note corrected to what is
  actually dead (trait name, kv put until wave 3)
- mem.rs: stale constructor doc line updated to the landed state
- tasks/gc-protocol.md: stray placeholder lines removed

Verification: cargo test (42 lib + 2 conformance + 3 git-interop),
cargo test --features mem and --all-features (97 lib + 37 conformance
+ 4 git-interop), clippy --all-targets and --all-features --all-targets
(-D warnings), fmt --check — all green post-fix. Wave 3 decomposition
unblocked.

src/backend.rs

@@ -30,9 +30,10 @@
 //! (ADR-002/006). GC state never rides this trait either (ADR-010 §2) —
 //! the crate-internal engine-state companion seam lives in [`crate::gc`].
 //!
-//! The `#![allow(dead_code)]` below marks the seam's scope: these items are
-//! the wave-1 contract surface, defined ahead of their drivers (the store
-//! core and engines of waves 2–5). It shrinks as those waves land.
+//! The `#![allow(dead_code)]` below still has real scope: `Backend`
+//! trait items with no in-tree caller yet (reporting names, the kv
+//! engines' whole-value `put` before wave 3) plus the wave-1 seam
+//! tests' doubles. It shrinks as the waves land.
 
 #![allow(dead_code)]