test: improve coverage to 98.2%, add staticcheck to CI (#7)

* 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

* fix(ci): remove Go 1.25 from matrix (go.mod requires 1.26)

.github/workflows/ci.yml

@@ -21,6 +21,11 @@ jobs:
         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'