fix(ci): re-pin rust-ci-reusable.yml past the toolchain-required-input fix (#217)

Pinned SHA predated `standards#439` (fix(rust-ci): pass explicit
`toolchain: stable` to SHA-pinned dtolnay action), so every run failed
at "'toolchain' is a required input" before any real check executed —
same class as squabbler's "Move 3: reusable-workflow pin drift".
Re-pinned to standards main tip (`7c9db0e5`), at/after #439.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -14,4 +14,4 @@ permissions:
 
 jobs:
   rust-ci:
-    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@d135b05bfc647d0c0fbfedc7e80f37ea50f49236
+    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@7c9db0e5909aab77bf11a444ade3b95c1d2b702e # re-pinned 2026-07-01: toolchain-required-input fix (standards#439)