fix(ci): status-gate — pass explicit toolchain: stable to rust-toolchain

The status-gate workflow added in #301 SHA-pins dtolnay/rust-toolchain but
omitted `with: toolchain: stable`. When the action is referenced by SHA the
`stable` ref's default is not inherited, so the step failed with
"'toolchain' is a required input" — the gate errored before ever running
status-gate.sh (a false-red, my own new bug). Add the explicit input.

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

Author: hyperpolymath

.github/workflows/status-gate.yml

@@ -36,6 +36,8 @@ jobs:
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@3c5f7ea28cd621ae0bf5283f0e981fb97b8a7af9 # stable
+        with:
+          toolchain: stable   # required input when the action is SHA-pinned (the `stable` ref's default is not inherited by SHA)
       - name: Cache cargo
         uses: Swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4 # v2
       - name: Run status-gate (proof + test count drift)