test: improve coverage to 98.2%, add staticcheck to CI

- Fix comment typos (Intensty → Intensity)
- Add tests for createStringerPalette modes (background fill, smart mode)
- Add tests for ColorString variants, BytesToColor determinism
- Add tests for GetBackgroundColor mid-tone edge case
- Add tests for GenerateOKLCHPalette negative/large inputs
- Add Go 1.25 to CI matrix for compatibility testing
- Add staticcheck lint step to CI
- Fix goimports formatting in hash_test.go

.github/workflows/ci.yml

@@ -14,13 +14,18 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ['1.26']
+        go-version: ['1.25', '1.26']
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5
         with:
           go-version: ${{ matrix.go-version }}
       - run: go vet ./...
+      - name: staticcheck
+        if: matrix.go-version == '1.26'
+        run: |
+          go install honnef.co/go/tools/cmd/staticcheck@latest
+          staticcheck ./...
       - run: go test -race -coverprofile=coverage.txt ./...
       - name: Upload coverage
         if: matrix.go-version == '1.26'