docs(tasks): decompose wave 4 — the postgres kv engine, the fleet host's full seam, the first SKIP-posture column

Five tasks against the wave-3 contract surface as reviewed (no
amendments from the gate):

- engine-postgres: the fleet-capable kv engine — the B5 row shape
  (fillfactor 90) in one engine-owned schema, the deadpool pool with
  per-connection prepared-statement discipline, the synchronous_commit
  parity knob wired per-connection, ON CONFLICT DO NOTHING CAS, the
  stable MVCC-snapshot list cursor, the batch-delete helper
  (key = ANY($1), consumer wave 6), the read-only virgin-store split
- engine-postgres-fleet: the engine-state seam's full clause families
  (ADR-012 §5's postgres row — every family yes): gc_pins + gc_window,
  the joint tx ownership rule in the JointTx shape, pin_commit
  atomicity, pin_renew/pin_check/arbitrate, the pg advisory lock as
  the sweep lease's shipped provider; fleet activation stays wave 6
- store-postgres-construction: KvEngine::Postgres slots in, the
  KvCursor arm, the dual-tier/kv-only constructors (DSN + opts +
  threshold + window delay), the non-fleet posture recorded, the
  pg-pure build verified
- suite-column-postgres: the first SKIP-posture column — the alkstore
  pg-factory pattern reused (fresh schema per open, CASCADE teardown,
  env-driven ALKBLOBS_PG_* settings), durable-schedule sweep rows,
  acceptance green against the harness server (pglo-poc :15432,
  verified live)
- review-wave-4: the engine-wave gate (engine-vs-contract conformance)

Verification: dependency graph validated (all ids resolve, no cycles,
topological order clean); docs-only change.

Refs: docs/plans/implementation.md §Wave 4

docs/plans/implementation.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: "2026-10-11 (wave 3 decomposed into tasks/)"
+last_updated: "2026-10-12 (wave 4 decomposed into tasks/)"
 ---
 
 # alkblobs — Implementation plan
@@ -51,7 +51,7 @@ ops payload surfaces; release readiness is last.
 | [1](#wave-1--foundations) | Crate/feature scaffold; hashing & keys (the one-way door); error model; Backend trait + I/O seams + engine-state seam shape; facade value types; conformance-suite convention | — | decomposed (2026-10-11) |
 | [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 | pending |
+| [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 |
 | [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 |