Add a dedicated clip header rail as the move handle

[ChuxiJ]

User Story

As a user, I want the clip's text strip to be the drag handle, so that I can move clips from the header and use the body for content-level interactions.

Problem

Clip text is only an absolute label today, not a separate interactive rail. Moving, slipping, and content inspection all compete for the same surface.

Root Cause

src/components/timeline/ClipBlock.tsx infers move or resize from one shared hit area, so there is no dedicated header drag handle and no safe body-only interaction region.

Solution

Split clip rendering into a header rail plus waveform body. Make the rail own move interactions and expose grab / grabbing cursor feedback, while leaving the body available for content-focused gestures.

Verification

• Add component coverage for the header rail cursor and ARIA affordance
• Manually verify header drag moves the clip
• Verify agent-compatible pointer automation can target the rail reliably
• Run npx tsc --noEmit
• Run npm run build

Notes

This issue covers the interaction-surface refactor and should preserve edge resize, fade, and warp handle behavior.