* 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)