ci: speed up Rust CI builds with prebuilt tools (#325)

* ci: speed up Rust CI builds with caching and prebuilt tools

The Windows builder takes ~6min vs ~3min on Ubuntu, largely because
cargo-llvm-cov is compiled from source on every run. This adds
Swatinem/rust-cache for build artifact caching and switches to
taiki-e/install-action for prebuilt cargo-llvm-cov binaries.

Also fixes the undefined `inputs.shell` references by hardcoding `bash`.

* fix: add Swatinem and taiki to cspell dictionary

* ci: remove rust-cache, no measurable benefit for this project

Investigated cache hit/miss across 3 CI runs. Even with warm cache
hits (macOS/Windows), build times were unchanged or slightly slower
due to cache download/extract overhead (~39MB) outweighing the small
dependency compile time savings.

.github/actions/rust_crate/action.yaml

@@ -13,34 +13,35 @@ inputs:
 runs:
   using: "composite"
   steps:
+    - name: Install cargo-llvm-cov
+      uses: taiki-e/install-action@cargo-llvm-cov
+
     - name: Check Formatting
       working-directory: ${{ inputs.working_directory }}
-      shell: ${{ inputs.shell }}
+      shell: bash
       run: cargo fmt --check
 
     - name: Clippy
       working-directory: ${{ inputs.working_directory }}
-      shell: ${{ inputs.shell }}
+      shell: bash
       run: cargo clippy --all-targets -- -D warnings
 
     - name: Build
       working-directory: ${{ inputs.working_directory }}
-      shell: ${{ inputs.shell }}
+      shell: bash
       run: cargo build --verbose
 
     - name: Test
       working-directory: ${{ inputs.working_directory }}
-      shell: ${{ inputs.shell }}
-      run: |
-        cargo install cargo-llvm-cov
-        cargo llvm-cov --lcov --output-path lcov.info
+      shell: bash
+      run: cargo llvm-cov --lcov --output-path lcov.info
     
     # Per https://stackoverflow.com/questions/73402042/github-action-expressions-split-string
     - name: Split package name
       env:
         PACKAGE_PATH: ${{ inputs.working_directory}}
       id: split
-      shell: ${{ inputs.shell }}
+      shell: bash
       run: echo "package_name=${PACKAGE_PATH##*/}" >> $GITHUB_OUTPUT
 
     - name: Upload Coverage