fix: Add required toolchain parameter to rust-toolchain action

The dtolnay/rust-toolchain action requires a 'toolchain' parameter (e.g., stable, nightly).
Added 'toolchain: stable' to both test and benchmarks jobs.

Previous error:
  'toolchain' is a required input
  Error: Process completed with exit code 1

Fixed in:
- test job (line 36)
- benchmarks job (line 235)

Author: claude

.github/workflows/ci.yml

@@ -33,6 +33,7 @@ jobs:
     - name: Setup Rust (for WASM)
       uses: dtolnay/rust-toolchain@0b1efabc08b657293548b77fb76cc02d26091c7e # master 2025-11-30
       with:
+        toolchain: stable
         targets: wasm32-unknown-unknown
 
     - name: Install ReScript
@@ -231,6 +232,7 @@ jobs:
     - name: Setup Rust
       uses: dtolnay/rust-toolchain@0b1efabc08b657293548b77fb76cc02d26091c7e # master 2025-11-30
       with:
+        toolchain: stable
         targets: wasm32-unknown-unknown
 
     - name: Install ReScript