CapRust 0.4.0-alpha.2 — transitions, batch undo, UX polish

Second alpha of the 0.4 line. Transitions on the timeline, batch
undo, and UX polish.

Added

• Transition easing. Clips carry transition_in_easing and
  transition_out_easing (both EaseCurve, default Linear). The
  clip properties panel shows an Easing dropdown under any
  transition that is present. The render currently emits plain
  linear fades because the gyan.dev essentials build we test
  against (2026-01-26) rejects fade=curve= with "Option not
  found". The field is stored and preserved for the day we pin a
  newer ffmpeg.
• Configurable transition duration. New
  Clip.transition_duration_ms (default 500) with a 100..3000 ms
  slider in the clip properties panel. Applies to both in and out
  edges of a clip.
• Xfade shifts followers. Attaching an xfade between two
  adjacent clips on the same track now moves the follower clip and
  every clip after it on that track left by the transition
  duration. Removing the transition shifts them back. Changing the
  duration shifts by the delta. All in one command so Ctrl+Z
  restores the whole batch. Applied in SetTransitionCommand and
  SetTransitionDurationCommand.
• Timeline overlap shading. Clips that carry an xfade on their
  in-edge show an accent-tinted slice with thin diagonal hatching
  over the overlap region. The crossfade footprint is visible at a
  glance.
• Fade-out on the last clip of an xfade run. The earlier
  clips' ends are consumed by the next clip's xfade, but the tail
  of the run can now fade back to black when its clip has
  transition_out == Some("fade").
• MacroCommand. Runs a list of commands as a single undoable
  step, with rollback on mid-batch failure. Used by the media bin
  batch drop so a 4-clip drop is one Ctrl+Z.
• Ctrl+Z / Ctrl+Y / Ctrl+Shift+Z keyboard shortcuts for
  undo/redo. Previously only reachable through the timeline
  toolbar icons.
• App icon embedded in the .exe via winres, runtime window
  icon, README header logo.
• Material-style egui_dock theme. Tab bar matches the panel
  fill, 30px tall, active tab uses an accent wash. Close button
  subtle, brightens on hover.
• Brighter toasts. Accent border, top highlight line, and a
  fill lift so notifications read as elevated.

Fixed

• Media bin multi-select and long-press drag. Ctrl+click
  toggles membership, plain click replaces. Holding the primary
  button for 500 ms on a card starts a drag; a shorter click only
  selects. The X button on a multi-selection removes every
  selected item at once.
• Batch drag to timeline. Dropping a multi-selection chains
  the clips left-to-right at the drop position and selects the
  result 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.
• Captions and text overlays seek-shift correctly. Their
  timeline_start_sec now follows the playhead seek, matching
  video and audio. Previously a seek into the middle of a long
  project left their enable windows on the original timeline.
• Apostrophes in caption text no longer break the preview.
  ASCII apostrophe and double quote substitute to U+2019 / U+201C
  at render time; both are safe inside a filtergraph single-quoted
  value.
• Media bin X button clickable on hover. The enclosing drag
  source claimed the pointer, so the button vanished mid-click.
  Rewritten to use the raw pointer position for visibility and the
  raw primary-click edge for the action.
• Total render duration accounts for xfade shortening. A
  3-second crossfade used to leave a 3-second black tail at the
  end of the render because the base and audio bed were sized to
  the unshortened sum.
• Overlapping clip detection. Run grouping now accepts a
  positive overlap equal to the transition duration, not just
  butt-joined clips.
• Dragging a clip that carries an xfade now removes the
  transition with a toast, restoring every follower to its
  original position. The user re-adds after the move.

Changed

• SetTransitionCommand rewritten: the shift is applied to the
  follower clip and every clip after it on the same track.
• Clip gained applied_xfade_shift_ms to track the shift the
  transition command applied.
• move_many::MoveManyCommand and macro_command::MacroCommand
  new core modules.
• VideoEncoder enum (H264/H265/AV1 x CPU/NVENC/AMF) added to
  ExportSettings in the previous alpha; unchanged here.

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.
• Transition easing stored but not rendered on the current ffmpeg
  build. Needs a build with fade=curve= support.
• 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.