Timeline: select several clips at once

[dvgmdvgm]

Empty space on a lane used to be a second way to drag the playhead. It is now
the selection surface.

The gestures

• Drag on empty lane space draws a rubber band and selects every clip it
  covers, on every lane it reaches.
• The playhead keeps its own surface — the ruler — and Shift+drag still
  marks the in/out range, on the lane and on the ruler alike.
• Ctrl+drag adds a band to what is already selected; Ctrl+CLICK on empty
  space still closes a gap, so nothing is taken away.
• Ctrl+click on a clip adds or removes it; Shift+click takes the whole
  rectangle between the first selected clip and the clicked one; a plain
  click on a member of a group narrows it back down to that clip.

What the selection can do. Move, delete, copy, split and link already
worked on the whole selection — the store has never been single-clip. Two
places had not caught up, and both are fixed here: deleting from a clip's
context menu first collapsed the selection to the clicked clip, and a group
dragged past t=0 piled onto zero instead of stopping there, because
setClipStarts clamps each clip on its own.

Three things worth reading in the diff

• The band's anchor is a TIME, not a screen position. The wheel scrolls and
  zooms the timeline mid-drag, and a pixel anchor would then sit at a
  different moment than the one that was grabbed. scripts/e2e44.mjs pins
  this: it scrolls the timeline in the middle of a band drag.
• A LOCKED track never enters a selection, not even through withLinked() —
  that helper adds an A/V twin without looking at the lock, while the store
  skips locked clips, so the pair would have come apart on the first move.
  unlocked() in src/engine/timelineSelect.ts is where that is enforced.
• windowDrag now also listens for pointercancel, for every drag in the
  file. A gesture cancelled by the system never sends pointerup, and the
  drag's overlay used to stay on screen and keep following the mouse with no
  button held.

Checks

• node scripts/check-select.mjs — the band, range and lock maths, pure
  node, 15 assertions. Written before the code and seen red.
• node scripts/e2e44.mjs — the gestures in a running app, 13 assertions.
  It uses TEXT clips only, so it needs no media and no ffmpeg, and it puts
  the track height back the way it found it.

Geometry lives in src/engine/timelineSelect.ts with no DOM in it, which is
what makes the first suite possible.

🤖 Generated with Claude Code