fix: polish GPUI editor corners and editing controls

[richiemcilroy]

The GPUI editor could paint waveforms and other timeline fills through rounded clip corners, and the sidebar tab strip covered its card corners. Keep those layers inside the rounded shape and paint the segment border last.

Make Split visible with a label, an active state, a scissors cut guide across segment tracks, and Escape to finish. Give automatic zoom generation a solid theme-aware button with a disabled loading state.

Validation: the isolated GPUI app and simulated merge with current main compile without warnings; waveform tessellation passes 64 clip-width, viewport and gain cases; rounded fade/hold construction, 25 split-preview boundary cases, the GPUI test-target typecheck, and scoped formatting pass. Five pre-existing Clippy findings remain outside the changed sections. Native appearance and interaction have not been tested; the shared desktop runtime is occupied by camera validation. Windows/Linux runtime validation is also pending.

Confidence Score: 5/5

The PR appears safe to merge, with the previously reported split-guide edge mismatch resolved.

Summary

• Prevents split guides from appearing too close to non-clip segment edges.
• Uses the same coordinate and duration calculations as actual split execution.
• Adds regression coverage for multiple non-clip track types and scrolled timelines.
• The broader PR also clips timeline artwork to rounded segment boundaries, updates split and automatic-zoom controls, and rounds the sidebar tab rail.

_{Reviews (2) · Last reviewed commit: "fix: align split previews with valid cut..."}

[richiemcilroy]

hey @greptileai, please re-review the PR