feat(tests): the conformance suite's local column — the dual-tier dispatch-coverage shape green (wave 2)

- tests/conformance/local_column.rs: the first dual-tier column, run
  against the kv = mem + large = local factory (ADR-012 §6.3) — fresh
  temp-root store per open, per-instance tracked roots, idempotent
  teardown safe against abandoned/panicking rows (structural tests
  included); never skips (mem-gated — the dual-tier constructor rides
  feature mem until wave 3 provides an ungated kv engine)
- the reference INV row set (rows.rs) mirrored against the dual-tier
  factory at a column-declared tuned threshold (1024), so the shared
  matrix and race fixtures exercise both tiers' bodies (INV-7)
- dispatch rows grown: pure-function routing across put paths and
  re-puts with an on-disk placement oracle (tier residency is
  facade-invisible; with kv = mem the medium is the oracle), threshold
  boundary legs at the tuned and shipped-default thresholds with the
  constructor-tuned re-route leg, and fall-through per op with
  both-tier miss shapes (ADR-003/ADR-012 §6.1/§6.3/§6.5)
- local-specific rows: on-disk stage-hygiene exact counts over both
  put paths (the real stage form, INV-3), pread windows over committed
  files with slice digests verified independently (ADR-006/ADR-010),
  virgin-store no-ops on a fresh root, and exact sweep counts over a
  both-tiers pool (INV-4/INV-5's both-tiers form)
- rows.rs shared fixtures made pub(crate); README records the column's
  row inventory (§Columns beyond the reference set), the declared-
  threshold posture, and the new current state
- stat-as-stat rides the mirrored INV-6 row's over-threshold legs;
  the Drop-cancellation hygiene form stays per-engine (recorded)

Verification: cargo test (default) green; cargo test --features mem
green (97 lib + 37 conformance + 4 git-interop); cargo test
--all-features green; clippy --all-targets -D warnings green in both
feature shapes (default + all-features); cargo fmt --check green;
cargo doc --no-deps shows only the pre-existing warning class

tests/conformance/main.rs

@@ -15,11 +15,17 @@
 //! skip-free reference shape) proves the contract implementable
 //! against the real type.
 //!
-//! The rows landed with wave 2's mem column: [`rows`] holds the
-//! reference row set (INV-1…6 plus the two constructor-mode rows,
-//! ADR-stamped, traced to [`backlog`]) and [`mem_column`] is the
-//! contract-reference column ([`README`](./README.md) §The reference
-//! row set is the recorded inventory other columns mirror).
+//! The reference rows landed with wave 2's mem column: [`rows`] holds
+//! the reference row set (INV-1…6 plus the two constructor-mode rows,
+//! ADR-stamped, traced to [`backlog`]); the columns' records live in
+//! their module docs ([`README`](./README.md) §The reference row set
+//! is the recorded inventory other columns mirror).
+//!
+//! The columns: [`mem_column`] — the contract-reference column;
+//! [`local_column`] — the suite's first dual-tier column (the
+//! `kv = mem` + `large = local` dispatch-coverage shape ADR-012 §6.3
+//! names), which mirrors the reference set and grows the
+//! dispatch-coverage and local-specific rows.
 //!
 //! Items beyond the tests below are the later columns' surface, hence
 //! the scoped `#![allow(dead_code)]` — it shrinks as columns land.
@@ -31,6 +37,8 @@ mod backlog;
 mod binding;
 mod factory;
 #[cfg(feature = "mem")]
+mod local_column;
+#[cfg(feature = "mem")]
 mod mem_column;
 mod rows;
 mod stamp;