chore: SHA-pin GitHub Actions for supply chain security

Pin all GitHub Actions to specific commit SHAs instead of mutable tags
to prevent supply chain attacks. Tags preserved as comments.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@4be9e76fd7c4901c61fb841f559994984270fce7 # stable
         with:
           components: rustfmt, clippy
       - uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
@@ -33,7 +33,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@4be9e76fd7c4901c61fb841f559994984270fce7 # stable
       - name: Install cargo-audit
         run: cargo install cargo-audit
       - name: Security audit
@@ -45,7 +45,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@4be9e76fd7c4901c61fb841f559994984270fce7 # stable
       - name: Install tarpaulin
         run: cargo install cargo-tarpaulin
       - name: Generate coverage