fix(ci): explicitly specify stable toolchain for clippy and doc checks

The workflow was failing because installing both stable and nightly
toolchains in the same job caused clippy to run with nightly by default
(the last installed toolchain becomes default).

Fixed by explicitly using +stable for clippy and doc commands, matching
the existing +nightly syntax for rustfmt.

Resolves GitHub Actions run #19350466507

Author: bug-ops

.github/workflows/ci.yml

@@ -51,10 +51,10 @@ jobs:
         run: cargo +nightly fmt --all -- --check
 
       - name: Clippy (all targets, all features)
-        run: cargo clippy --all-targets --all-features --workspace -- -D warnings
+        run: cargo +stable clippy --all-targets --all-features --workspace -- -D warnings
 
       - name: Check documentation
-        run: cargo doc --no-deps --all-features --workspace
+        run: cargo +stable doc --no-deps --all-features --workspace
         env:
           RUSTDOCFLAGS: "-D warnings"