fix(crdt): allow match_same_arms, and record why the gate missed it

The enumeration is the contract. Clippy's `match_same_arms` would
collapse the three `Ok(())` rows into one alternation, which is exactly
the conflation this lane exists to remove --- it would stop the table
from showing that `(forward, empty, None)` and `(history, empty, Some)`
are valid for OPPOSITE reasons.

The lint reached CI rather than the gate because the gate's clippy step
runs default features, so `#[cfg(feature = "crdt")]` code is never
linted locally. Five consecutive green gate runs could not see it. The
gap is recorded in the ledger; fixing it means adding a second clippy
flavor to shared gate infrastructure, which is its own lane.

docs/active-work.md

@@ -337,6 +337,24 @@ of such an edit.
   machine state all varied too, and a `BrokenPipe` on a socket handshake
   is exactly what those can drive.
 
+**A GATE COVERAGE GAP, found the expensive way.** The local gate's
+clippy step is `cargo clippy --workspace --all-targets -- -D warnings`
+— **default features**, so **no `#[cfg(feature = "crdt")]` code is ever
+linted locally**. CI lints it (`--no-default-features --features
+luajit,crdt`), so a crdt-only lint passes eight green gate stages and
+then reds `Test (crdt)`. That is what happened here: a
+`clippy::match_same_arms` on the new enumeration, invisible to five
+consecutive local gate runs.
+
+The lint itself is `#[allow]`ed with a reason — collapsing the three
+`Ok(())` arms is exactly the conflation this lane removes, and would
+hide that `(forward, empty, None)` and `(history, empty, Some)` are
+valid for opposite reasons. **The gap is not fixed here**: adding a
+second clippy flavor to `scripts/gate` is a change to shared
+infrastructure and belongs in its own lane, alongside U9's still-unrun
+discriminating control. Recorded so the next lane touching
+crdt-gated code does not rediscover it at CI.
+
 **Two registry rows gained occurrences on this lane**: R7's eighth (the
 green/red pair whose heads differ by one markdown file) and **U6's
 second — the first time U6 has ever reproduced**, twice in a row, and

src/buffer.rs

@@ -3025,6 +3025,14 @@ mod tests {
         /// `(forward, empty, Some)` row is **unreachable from any
         /// generated forward input**, because a forward empty form
         /// short-circuits and a forward real-delta form is not empty.
+        // The enumeration IS the contract. `match_same_arms` would have
+        // the three `Ok(())` rows collapsed into one alternation, which
+        // is exactly the conflation this lane exists to remove: it would
+        // stop the table from showing that `(forward, empty, None)` and
+        // `(history, empty, Some)` are valid for OPPOSITE reasons, and a
+        // future reader would have no way to see which quadrant a change
+        // moved.
+        #[allow(clippy::match_same_arms)]
         fn check_crdt_op_shape(
             class: OperationClass,
             edit: &Edit,