feat(tests): the conformance suite's postgres column — the first SKIP-posture column green (wave 4)

- The column (tests/conformance/postgres_column.rs, feature `postgres`,
  default-off): the factories are the alkstore pg-factory pattern reused —
  `dual-tier postgres+local` (kv = postgres + large = local at the
  column-declared threshold 1024, window delay `Duration::ZERO`) and
  `kv-only(postgres)` at 1024 (the rejection row's host); a fresh
  engine-owned schema per `open()` (unique per call, seeded per
  tag/pid/time — racing opens included), registry-tracked per instance,
  teardown `DROP SCHEMA IF EXISTS … CASCADE` over exactly the minted
  schemas + the vended temp roots (idempotent, owned, panicking-row-safe)
- The SKIP posture: connection settings ride the environment
  (ALKBLOBS_PG_HOST/PORT/USER/PASSWORD/DB), every runner gates behind a
  reachability probe, server-less runs skip cleanly (a skip is not a
  failure); the acceptance run is green against the harness server
  (dockerized `pglo-poc`, :15432 — the alkstore harness's same server)
- Rows: the form-independent reference rows mirror verbatim (INV-1,
  INV-6, the NoLivenessSources/ProtectFailed abort shapes, the kv-only
  rejection row); the sqlite column's durable-schedule sweep family
  mirrors (schedule-relative INV-2/INV-3 readings, the two-sweep exact
  counts over the full protection set, the race across the window
  delay, the holder-stages-only lease shape, INV-5 list correctness);
  the dispatch rows adapt the placement oracle by medium (independent-
  connection counts of the schema's rows vs the root's committed files)
  plus the virgin-store row; every row ADR-stamped, traced to the
  backlog
- Structural factory tests: fresh isolated opens (two opens mint
  distinct schemas and share nothing) and teardown against a panicking
  row (schemas really dropped, pg_namespace-verified)
- README amended: the postgres rows table in §Columns beyond the
  reference set; the column plan's SKIP-posture note names the landed
  column; current state (four columns, waves 2–4)

Verification: cargo test (default) 69/19/3 ok — the column compiles
out; cargo test --features postgres 102/36/3 ok against the harness
server (and green skipping server-less); cargo clippy --all-targets --
-D warnings green (default, postgres, all-features); cargo fmt --check
green; pg_namespace verified schema-clean after live runs.

tests/conformance/main.rs

@@ -30,7 +30,10 @@
 //! shape, `kv = sqlite` + `large = local`), which mirrors the
 //! form-independent reference rows and grows the durable-schedule
 //! sweep rows (README §The schedule-relative reading for SQL-backed
-//! columns).
+//! columns); [`postgres_column`] — the suite's first SKIP-posture
+//! column (the fleet-capable-kv shape, `kv = postgres` + `large =
+//! `local`, feature `postgres` default-off), which mirrors the same
+//! sets against the harness server and skips cleanly server-less.
 //!
 //! Items beyond the tests below are the later columns' surface, hence
 //! the scoped `#![allow(dead_code)]` — it shrinks as columns land.
@@ -45,6 +48,8 @@ mod factory;
 mod local_column;
 #[cfg(feature = "mem")]
 mod mem_column;
+#[cfg(feature = "postgres")]
+mod postgres_column;
 mod rows;
 #[cfg(feature = "sqlite")]
 mod sqlite_column;