fix(ci): drop beta-Rust CodeQL leg (cancelled/hanging 'checks 2 things forever') (#375)

## What

The CodeQL matrix analysed `actions` + `rust`. **CodeQL Rust support is
beta** — that leg cancels/hangs and never resolves (the recurring
*"CodeQL checks 2 things forever"* symptom seen on this repo's Actions
tab), so it provides **zero real security coverage** while looking
permanently in-progress.

## Fix

Remove the `rust` leg, keep `actions` (this repo's primary analysable
surface). Matrix stays non-empty → no zero-jobs `startup_failure`.

## Rust security (follow-up)

Rust belongs in native tooling, not CodeQL. `rust.yml` already runs
clippy + build + test. Recommended next: a **cargo-audit** gate —
verified locally that it runs and already flags a real vuln:
`crossbeam-epoch 0.9.18` →
[RUSTSEC-2026-0204](https://rustsec.org/advisories/RUSTSEC-2026-0204)
(fix `>=0.9.20`), plus `anyhow 1.0.102` unsound
([RUSTSEC-2026-0190](https://rustsec.org/advisories/RUSTSEC-2026-0190)).

## Verification

`actionlint` exit 0; matrix = 1 `actions` leg, 0 active `rust` legs.

## Context

Estate-wide shared-fate: **13 repos** carry this beta-Rust CodeQL leg.
Reference fix for the sweep.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/codeql.yml

@@ -38,8 +38,6 @@ jobs:
           # the prior matrix as containing no language present in the repo.
           - language: actions
             build-mode: none
-          - language: rust
-            build-mode: none
 
     steps:
       - name: Checkout