docs(tasks): correct the wave-5 pure-build gates — the literal kv-less pg-lo config is not a gate

The wave-5 engine task (d69d9a3) found the literal
--no-default-features --features pg-lo config cannot compile the whole
crate: with no kv feature enabled, KvEngine/KvCursor go empty (E0004)
and the facade's by-reference dispatch matches are non-exhaustive
(references are always considered inhabited). Structural, not a bug —
per ADR-011 §4 every constructor mode pairs a kv engine, so a build
with zero kv engines is not a supported configuration (ADR-013 §4's
supported non-default configurations all include one).

The meaningful pure builds, verified on this tree:
- --no-default-features --features "pg-lo,mem" — check/test/clippy
  green (115 tests ok server-less)
- --no-default-features --features "postgres,pg-lo" — check/clippy
  green, test target compiles

Amended: engine-pg-lo (description + acceptance + gates),
engine-pg-lo-window (acceptance), store-pg-lo-construction (the full
structural reading + acceptance), review-wave-5 (gates). The engine
task's Notes/Summary already recorded the finding; the criteria text
now matches what was verified.

Verification: cargo test 69/19/3 ok, fmt --check green (docs-only
change).

tasks/review-wave-5.md

@@ -75,9 +75,12 @@ Gate checks:
   `#![allow(dead_code)]` scope notes matching their remaining scope.
 - **Gates green**: `cargo test` (default), `cargo test --features
   pg-lo` (server-less: skipping), `cargo test --features "pg-lo,mem"`,
-  `cargo test --all-features` (against the server), the pg-lo-pure
-  checks (`cargo check --no-default-features --features pg-lo` and
-  `--features "pg-lo,mem"`), `cargo clippy --all-targets -- -D
+  `cargo test --all-features` (against the server), the pg-lo pure
+  builds (`cargo check --no-default-features --features "pg-lo,mem"`
+  and `--features "postgres,pg-lo"` — the literal kv-less
+  `--features pg-lo` config is not a gate: with no kv engine the
+  facade's `KvEngine`/`KvCursor` go empty, E0004; see
+  `store-pg-lo-construction`), `cargo clippy --all-targets -- -D
   warnings` (default + the pg-lo variants + `--all-features`),
   `cargo fmt --check`, `cargo doc --no-deps`.