feat(tests): the conformance suite's pg-lo column — the LO-medium SKIP-posture column green (wave 5)

- tests/conformance/pg_lo_column.rs (new): the two pair-gated factories
  (DualTierMemPgLoFactory on all(mem, pg-lo) — the one-window-host rule's
  named test topology; DualTierPostgresPgLoFactory on
  all(postgres, pg-lo) — the consolidation posture, two schemas per open,
  carrying the dormancy row), a fresh dedicated database per open with
  the engine-owned schemas minted inside, teardown reaping tracked LOIDs
  (lo_unlink) then dropping the minted schemas then the database
  (WITH (FORCE)) with per-open drop-guard janitors for the abandoned
  paths, and the rows: the verbatim form-independent mirror (INV-1, INV-6,
  the two no-op abort shapes), the schedule-relative INV-2/3, the
  two-sweep exact counts over the full protection set across each
  factory's observed window host, the window-delay race, the
  holder-stages-only lease, INV-5's durable row, the three dispatch rows
  over the LO-medium placement oracle, the INV-1 extension (window-edge
  ranged reads + the planted-overclaim beyond-length clean failure), the
  virgin-store rows, and the dormancy row; structural isolation +
  panicking-row tests probing zero orphan residue (no kv-only rejection
  row — recorded; pg-lo is a large engine)
- the LO-isolation corollary carried in the engine task's recorded
  per-test-database shape (the C7 lesson): zero LO residue on the shared
  harness database is structural, so the engine suite's
  shared_harness_db_holds_zero_lo_residue gate stays green alongside the
  column under --all-features
- main.rs: the pg_lo_column gate (feature pg-lo, default-off) + the
  columns' doc paragraph
- README: the pg-lo rows table in §Columns beyond the reference set, the
  SKIP-posture note naming the second landed column and the per-factory
  feature-pair gating, current state = five columns
- store.rs: one mechanical gate fix — the PgOpts import was unused in the
  sqlite+pg-lo config (tripping clippy --features pg-lo -D warnings);
  the gate now names the configs that consume it

Verification (harness server live 2026-10-12): --all-features against the
server fully green (188 lib + 106 conformance + 4 git-interop; zero
LO/schema/database residue after the run); default test green (69/19/3);
--features "pg-lo,mem" and "postgres,pg-lo" green server-less
(skipping cleanly) and --features pg-lo alone; clippy -D warnings green
on default, pg-lo, pg-lo+mem, postgres+pg-lo, all-features; fmt --check
green; cargo doc --no-deps clean.

tests/conformance/main.rs

@@ -33,7 +33,12 @@
 //! 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.
+//! sets against the harness server and skips cleanly server-less;
+//! [`pg_lo_column`] — the suite's second SKIP-posture column (the
+//! LO-medium column, feature `pg-lo` default-off, its two factories
+//! spanning feature pairs `mem`+`pg-lo` and `postgres`+`pg-lo`), which
+//! runs the same sets against the LO medium 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.
@@ -48,6 +53,8 @@ mod factory;
 mod local_column;
 #[cfg(feature = "mem")]
 mod mem_column;
+#[cfg(feature = "pg-lo")]
+mod pg_lo_column;
 #[cfg(feature = "postgres")]
 mod postgres_column;
 mod rows;