feat(tests): the conformance suite's fleet column — the exact-count fleet sweep outcomes under concurrency, green against the harness server (wave 6)

- tests/conformance/fleet_column.rs (cfg feature = "postgres"; the
  consolidation pieces cfg the postgres+pg-lo pair): the fleet-validity
  rows, the joint entry+pin tx rows (both-or-neither over an
  independent connection; the dedup re-put's upsert-renew; the second
  instance's re-put composing a second row — protection by row; the
  pg-lo consolidation round-trip byte-exact over the shared instance),
  the TTL/renewal rows (the guard's TTL/3 cadence through the short
  test TTL; the never-resurrect semantics through the facade; the
  paused holder's expiry boundary with the delete-then-recover
  backstop), and the exact-count fleet sweep rows — the wave's gate,
  INV-4's engine-gated fleet form: the two-instance exact-count shape,
  the lease contention row (SweeperLock as report data, nothing
  deleted), the cross-instance arbitration row (GcRefuse until the
  foreign row's expiry), the race across the window delay closed by
  the row, the maintenance step's exact reap, and the fleet liveness
  verb row (the schema-ownership rule asserted; the deregistration
  correction flow); the kv-only rejection row mirrored verbatim
  against the fleet kv-only factory
- three fleet factories after the postgres column's registry pattern
  extended to the fleet shape — one minted schema, two Store
  instances over it, each its own owner id (the pin rows compose by
  owner); the dual-tier instances carry private local roots riding
  FleetSharedMedia::asserted; the consolidation factory follows the
  pg-lo column's LO-isolation corollary (a TrackedDb::new constructor
  added to pg_lo_column.rs for the private janitor field); teardown
  drops exactly the minted schemas — idempotent, owned,
  panicking-row-safe, structurally exercised
- the SKIP posture rides the postgres column's reachability probe:
  server-less runs skip cleanly, acceptance is green against pglo-poc
  (:15432) under --features postgres with the consolidation rows
  under --all-features; the test tuning is the window delay 0 and the
  short test pin TTL (the fleet constructors' parameter) — ordering
  rides the tunings or the manual-expiry instrument, no sleep-based
  race assertions
- README amended: the fleet rows table (§Columns beyond the reference
  set), §Column plan (the fleet column is the wave-6 gate's host),
  §Current state — six columns
- task suite-column-fleet completed (Notes + Summary recorded,
  including the recorded deviations: the structural predicate shapes
  reach no fleet constructor; the cross-instance mismatch leg is
  unexercisable on the one-instance harness server — the probe's
  errors-closed posture asserted instead)

Verification: cargo test (default) green; cargo test --features
postgres green server-less (skipping) and green against the harness
server; cargo test --all-features green against the server (125
conformance + 226 library tests); cargo clippy --all-targets
-- -D warnings and cargo fmt --check green on the default, postgres,
and all-features variants

tests/conformance/main.rs

@@ -38,7 +38,13 @@
 //! 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.
+//! server-less; [`fleet_column`] — the suite's third SKIP-posture
+//! column (the fleet column — the exact-count fleet sweep outcomes
+//! under concurrency, wave 6's gate, feature `postgres`-gated, its
+//! consolidation factory also gated on `pg-lo`), which runs the
+//! fleet-validity rows, the joint-tx rows, the TTL/renewal rows, and
+//! the engine-gated fleet-form sweep rows against two store instances
+//! over one shared pool 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.
@@ -49,6 +55,8 @@ mod backlog;
 #[cfg(feature = "mem")]
 mod binding;
 mod factory;
+#[cfg(feature = "postgres")]
+mod fleet_column;
 #[cfg(feature = "mem")]
 mod local_column;
 #[cfg(feature = "mem")]