CapRust 0.4.0-alpha.1 — editor interaction pass

Fourth public alpha. Editor interaction pass: media bin multi-select,
group drag on the timeline, and the caption pipeline.

Added

• Media bin multi-select. Ctrl+click toggles membership, plain
  click replaces. Selected cards get a thicker accent border. The X
  button on a card that belongs to a multi-item selection removes
  every selected item at once, with Ctrl+Z restoring the clips that
  referenced them.
• Long-press drag from the media bin. Holding the primary button
  on a card for 500 ms starts a drag; a shorter click only selects.
  The card preview badge follows the cursor and shows how many items
  are being dragged ("1 clip" / "N clips").
• Batch drag to the timeline. Dropping a multi-selection chains
  the items left-to-right at the target drop position, using each
  item's duration. The inserted clips come back selected, so a
  follow-up move or delete acts on the whole batch.
• Group move on the timeline. Ctrl+click or marquee selects
  multiple clips; dragging one of them moves the entire group by the
  same time and track delta in a single undoable step. Previously a
  multi-select drag moved only the clip under the pointer.

Fixed

• Captions and text overlays rendered at the wrong time after a
  seek. Video and audio already shifted their timeline_start_sec
  by seek_sec and skipped clips before the seek. Captions and text
  overlays did not, so a seek into the middle of a long project left
  their enable windows on the original timeline. Same seek
  arithmetic now applies to both arms.
• Apostrophes in caption text broke the preview. Both \\' and
  the shell-style '\\'' escape fail on the BtbN / gyan ffmpeg
  builds; ffmpeg's filter option parser does not implement
  close-reopen semantics. The first leaks the rest of the option
  list as a new filter name (the classic 'No such filter: '
  error), the second terminates the text value early and renders
  fragments of the filtergraph on the frame. Fix: substitute ASCII
  apostrophe and double quote with U+2019 / U+201C at render time;
  these are not filtergraph special chars and never need escaping.
• Media bin X button was unclickable on hover. The button was
  only drawn when the card was hovered, but the enclosing
  dnd_drag_source claimed the pointer the moment it entered the
  card. egui's hover arbitration then reported the card as
  not-hovered as soon as the pointer reached the X, so the button
  vanished mid-click. Rewrote visibility and hit test against the
  raw pointer position.
• Every plain click in the media bin started a drag. egui's
  dnd_drag_source uses the whole available UI rect as its interact
  area, so inside a scrollable grid every click anywhere in the
  panel started a drag on the last-registered card. Replaced with
  per-card allocate_exact_size + manual DragAndDrop::set_payload.

Changed

• last_dnd_payload, pending_drop and the timeline drop handler
  now carry Vec<Uuid> instead of a single Uuid.
• MediaBinState gained selected_media: Vec<Uuid>.
• New core module commands::move_many with MoveManyCommand.

Known issues

• No installer yet; download the nightly ZIP and extract it.
• FFmpeg is not bundled. First run offers to download a BtbN build
  or to point at an existing install.
• Windows only.
• A batch drop emits one RippleInsertCommand per item; undo takes
  N presses. A follow-up will collapse them into one command.
• Text overlay rotation is stored in the model but neither rendered
  nor exposed in the UI.
• Preview A/V sync has a residual 100-300 ms lead/lag on some
  hardware. Tunable via CAPRUST_AV_DELAY_MS.