docs(review): the wave-3 review gate — every gate check green

Reviewed the wave-3 commits (engine-sqlite, engine-sqlite-window,
store-sqlite-construction, store-sweep-durable-window,
suite-column-sqlite) against the cited ADR/spec text line by line:
ADR-003/007/008 §2, ADR-010 §2, ADR-012 §1/§5/§6.7, store-api.md,
implementation.md §Review gates — the engine-vs-contract conformance
(suite column green server-less in the default build; the window
family contract-tested per its applicability row; virgin-store
semantics incl. the no-tables-yet form; the size metadata-only
discipline proven index-riding), the durability posture (WAL +
synchronous=NORMAL verified at open; the single-writer ceiling posture,
not hedge), the durable sweep form vs ADR-008 §2/ADR-012 §6.7 (stage at
mark_end + window; next-sweep execution with raced execution-time
re-arbitration; schedule-relative exact counts; aborts stage and
execute nothing), the seam reduction (gc_window only; no pin rows, no
sweep-lock body, no EngineFleetState), and the blocking discipline
(all rusqlite in spawn_blocking). Named design points audited as
resolved; family conventions clean; no wave-4 amendments arise.

Inline fixes (review-of-record):
- src/gc.rs: the seam's window_stage doc retired the wave-1
  lazy-create hedge for the shipped constructor-at-open posture
  (a tableless database surfaces the engine error on stage;
  per-engine tolerances are each engine's recorded posture)
- tasks/engine-sqlite-window, store-sweep-durable-window,
  suite-column-sqlite: stray placeholder lines above the filled
  Notes/Summary removed (the wave-2 gate's same cleanup)
- tests/conformance/README.md: trailing newline added

Verification (all re-run green after the fixes):
- cargo test (default): 69 lib + 19 conformance + 3 git-interop
- cargo test --features mem / --all-features: 124 lib + 54
  conformance + 4 git-interop each
- cargo clippy --all-targets -- -D warnings (default + all-features)
- cargo fmt --check
- cargo doc --no-deps at the pre-existing warning baseline (26)

Wave 4 (postgres) decomposition is unblocked.

tests/conformance/README.md

@@ -360,4 +360,4 @@ feature-gated on `sqlite` — **default-on**, so the column runs in the
 default build: every `cargo test` run exercises it. The sqlite
 engine's no-tables-yet read tolerance is engine-instrumented
 (src/sqlite.rs's tests; the constructor-vs-lazy timing design point
-there), the column's virgin-store row being its facade form.
+there), the column's virgin-store row being its facade form.