feat: add focused clip range slicing

[ChuxiJ]

Summary

• add sliceClipToRange to the project store for focused clip isolation
• wire clip body drag to preview and commit the selected range on mouse-up
• cover audio zero-cross snapping, MIDI grid snapping, and component drag behavior

Linked Issues

• Closes Auto-slice the focused clip to the dragged in-clip selection #684

Verification

• npx vitest run src/store/__tests__/sliceClipToRange.test.ts tests/unit/clipRangeSlice.test.tsx tests/unit/clipBlockHover.test.tsx tests/unit/clipResizeAndFadeVisuals.test.tsx tests/unit/clipResizeModifiers.test.tsx tests/unit/clipScissorMode.test.tsx tests/unit/clipWaveform.test.tsx
• npx tsc --noEmit
• npm run build
• Headless browser QA against http://127.0.0.1:4173/ verifying body drag slices the focused clip into 3 segments and header drag still moves the selected segment

Notes

This PR is stacked on top of #686. Review this diff against feat/v0.1.1-clip-surface-ux.