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/store-pg-lo-construction.md

@@ -103,12 +103,25 @@ the durable window gains its large-engine host arm.
   proof). Env-driven (`ALKBLOBS_PG_*`), skipping cleanly server-less
   (the engine tests' convention); the janitor discipline rides from
   the start (amendment A2).
-- **The pg-lo-pure builds** — `cargo check --no-default-features
-  --features pg-lo` (engine + window family compile, constructors
-  cfg'd out where `mem` is absent) and
-  `cargo test --no-default-features --features "pg-lo,mem"` (the
-  mem-pglo constructor constructible): ADR-013 §4's supported
-  non-default configurations; verify both compile clean.
+- **The pg-lo pure builds** — the supported non-default configurations
+  are `cargo check/test --no-default-features --features "pg-lo,mem"`
+  (the one-window-host topology's build; the mem-pglo constructor
+  constructible) and `cargo check --no-default-features --features
+  "postgres,pg-lo"` (ADR-013 §4's pg-pure consolidation build); verify
+  both compile clean. **The literal kv-less config
+  (`--no-default-features --features pg-lo`) is not a gate and cannot
+  compile the whole crate** — do not burn time on it: 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, so an empty enum behind
+  `&` still requires arms). This is 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 shared pieces
+  themselves (pg_common, the engines) compile in the literal config —
+  only store.rs errors. The finding is the wave-5 engine task's
+  (d69d9a3, recorded in its Notes); this task's verification uses the
+  two meaningful builds above.
 
 ## Acceptance Criteria
 
@@ -128,8 +141,10 @@ the durable window gains its large-engine host arm.
       branch keys off the engine representation, never a flag
 - [ ] The connection-settings design point (two DSN+opts pairs; the
       consolidation corollary) decided and recorded
-- [ ] The pg-lo-pure builds (`--no-default-features --features pg-lo`
-      and `--features "pg-lo,mem"`) compile clean
+- [ ] The pg-lo pure builds (`--no-default-features --features
+      "pg-lo,mem"` and `--features "postgres,pg-lo"`) compile clean
+      (the literal kv-less config is not a gate — see the Description
+      bullet)
 - [ ] `cargo test` (default) green; `cargo test --features pg-lo` and
       `--features "pg-lo,mem"` green server-less (skipping);
       `cargo clippy --all-targets -- -D warnings` and `cargo fmt