docs(tasks): decompose wave 5 — the pg-lo large engine, the window host's large-tier arm, the LO-medium SKIP-posture column

- engine-pg-lo: the Backend impl behind feature pg-lo (default-off) —
  companion-table authority, one-tx staged put at the 512 KiB knee,
  descriptorless lo_get windows, the orphan-recovery sweep, the
  posture deltas recorded; the shared-helper/PgOpts/LO-isolation
  design points named (LOs are database-level — the C7 lesson cited
  so it is not rediscovered)
- engine-pg-lo-window: the EngineWindowState impl per ADR-012 §5's
  pg-lo row — gc_window only, no fleet family anywhere
- store-pg-lo-construction: LargeEngine::PgLo slotting (the large
  sub-enum design point), the consolidation + mem-pglo constructors
  (the fleet-valid set completed, implicitly non-fleet), and the
  one-window-host rule's store-core completion (dormancy + the
  large-host sweep arm)
- suite-column-pg-lo: the second SKIP-posture column, two factories
  spanning feature pairs, the dormancy row, zero LO residue in
  teardown
- review-wave-5: the engine-wave gate; amendment A2 (janitor
  adoption) verified here

Harness server cited in every server-touching task: dockerized
pglo-poc (postgres:16-alpine) on :15432, db blobs, postgres/poc
(verified live 2026-10-12); env-driven ALKBLOBS_PG_* settings, clean
server-less skips, acceptance requires green against the server.

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

docs/plans/implementation.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: "2026-10-12 (wave 4 decomposed into tasks/)"
+last_updated: "2026-10-12 (wave 5 decomposed into tasks/)"
 ---
 
 # alkblobs — Implementation plan
@@ -52,7 +52,7 @@ ops payload surfaces; release readiness is last.
 | [2](#wave-2--the-store-works-dependency-free-mem--local) | mem + local engines; facade put/read paths + dispatch + constructor modes; in-process GC protocol; suite's first columns | wave 1 | decomposed (2026-10-11) |
 | [3](#wave-3--sqlite-engine) | sqlite kv engine (default): WAL posture, prepared statements, engine-state window family, suite column | wave 2 | decomposed (2026-10-11) |
 | [4](#wave-4--postgres-kv-engine) | postgres kv engine: schema, pool, CAS rows, stable-cursor list, full engine-state clause families, suite column | wave 2 | decomposed (2026-10-12) |
-| [5](#wave-5--pg-lo-large-engine) | pg-lo large engine: LO staged put, descriptorless lo_get windows, companion rows, window-family seam clauses, suite column | wave 4 | pending |
+| [5](#wave-5--pg-lo-large-engine) | pg-lo large engine: LO staged put, descriptorless lo_get windows, companion rows, window-family seam clauses, suite column | wave 4 | decomposed (2026-10-12) |
 | [6](#wave-6--fleet-gc) | Fleet GC mechanism: DB-backed pins, joint entry+pin tx, sweep lease, SQL delete arbitration, fleet-validity predicate + constructor declarations | waves 4 + 5 | pending |
 | [7](#wave-7--ops-surface) | `ops` feature: alkcall-backed op family, ACL mapping, pin tokens + renewal, fanout seam | wave 6 | pending |
 | [8](#wave-8--fuzzing) | Fuzzing: family pattern targets (key encoding, ops payloads), corpus-replay gate | wave 7 | pending |