Fix the ktlint and compat-test failures, and start using the Rust gate

Two more from the step-2 removals:

**Two unused Kotlin imports** — `FilterChip` (the Note/List switch) and
`Icons.Filled.Create` (the "switch to a note" icon), both orphaned when their
callers went. ktlint treats them as errors.

**`server_info_tolerates_unknown_and_absent_fields`** pinned
`sync_protocol_version: 1` as a literal, so bumping the protocol to v2 made it
fail for a reason that has nothing to do with what it tests. It is about unknown
FIELDS; the versions now come from `CLIENT_PROTOCOL_VERSION`, like every other
test in that file already did.

The bigger fix is the habit. `ci-requirements.md` has documented since
2026-08-18 that the operator authorised running fmt/clippy/test against the CI
image locally, and I had not been doing it. All three now pass here — 116 tests,
clippy clean, fmt clean — and every Rust failure in this milestone so far would
have been caught by them in under a minute instead of by CI, several commits
downstream. Noted in ci-requirements so the next session doesn't relearn it: a
removal is exactly the change that looks too safe to check.

ci-requirements.md

@@ -305,6 +305,15 @@ matched CI run 3931's byte for byte. Same image, same lockfile, same units.
 take seconds (~30s for clippy). It is gitignored and reaches ~1.4 GB; delete it
 whenever the space is wanted.
 
+**Run these on every Rust-touching push, not just the ones that feel risky.** Four
+consecutive failures across M13's removals — a private `fn` deleted along with the
+`pub fn` above it, an orphaned `#[serde]` attribute left where a field was removed,
+and a test pinning a protocol version literal — were all caught by these three
+commands in under a minute each, after CI had already found them the slow way. A
+removal is exactly the kind of change that looks safe and isn't: nothing in the
+Python or TypeScript lanes compiles Rust, so a break can travel several commits
+before the first lane that does gets to it.
+
 **Don't infer formatting from existing code.** Several lines in `local/store.rs`
 exceed 100 characters and survive only because rustfmt cannot break a string
 literal — copying that shape caused one of four consecutive fmt-only CI failures,