ci: optimize code coverage performance from 13min to ~3-5min

## Performance Optimizations

### Major Improvements
- **Use taiki-e/install-action** instead of cargo install for tarpaulin
  - Pre-built binaries vs compiling from source (~5-7min savings)
  - Cached installation for subsequent runs

- **Add cargo caching** to coverage job
  - Use Swatinem/rust-cache@v2 like test jobs
  - Avoid recompiling dependencies every run (~2-3min savings)

- **Add timeout protection** to prevent hanging
  - 300 second timeout for coverage generation
  - Better failure handling for long-running tests

### Expected Results
- **Before**: ~13 minutes (cargo install + full recompile)
- **After**: ~3-5 minutes (cached tools + dependencies)
- **Improvement**: 60-75% faster coverage runs

### Technical Details
- taiki-e/install-action uses pre-built binaries when available
- Cargo cache reuses compiled dependencies across runs
- Timeout prevents CI from hanging on problematic tests

This should significantly improve developer experience by reducing
PR feedback time while maintaining the same comprehensive coverage.

Author: joshrotenberg

.github/workflows/ci.yml

@@ -54,11 +54,16 @@ jobs:
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
 
+      - name: Cache cargo
+        uses: Swatinem/rust-cache@v2
+      
       - name: Install tarpaulin
-        run: cargo install cargo-tarpaulin
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-tarpaulin
 
       - name: Generate coverage
-        run: cargo tarpaulin --workspace --all-features --out xml
+        run: cargo tarpaulin --workspace --all-features --out xml --timeout 300
 
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v3