feat: add interactive live preview server with web UI

[AsPulse]

Adds tellur-live, an interactive preview server that hot-reloads
timeline cdylib plugins and streams frames to a bundled React/Vite
web UI. Lets you iterate on a timeline without recompiling the host
binary, with a scrub-precise frame API alongside h264 segment streaming
for smooth playback.

• tellur-live (new crate): plugin host (libloading-based hot-reload,
  filesystem-watch rebuild), HTTP server with a small REST surface, and
  a CLI binary. export_timeline!("id", "Title", build_fn) /
  export_timeline_collection! macros expose a TimelineCollection
  from a cdylib.
• REST API: /api/info, /api/frame (single PNG / RGBA), /api/stream
  (chunked frame stream for scrubbing), /api/video.mp4 (h264 for
  smooth playback), /api/events (SSE).
• Web UI (React/Vite, bundled into the cdylib): preview scrubber,
  transport (play / pause / loop / speed), timeline editor with viewport
  bar, resolution + FPS presets, automatic cache invalidation on
  plugin reload. Hybrid playback path keeps h264 for playback and RGBA
  for scrub.
• Demo plugin (examples/demo_timeline_plugin.rs cdylib + shared
  demo_scene/ module): a 7.6 s instrument-themed motion-graphics piece
  across four scenes (OVERTURE / FIELD / SCAN / RESOLVE) with a
  persistent HUD scaffold, layered DropShadow halo + drop, and a cached
  Hud VectorComponent whose Hash/Eq is driven by Phase + a section
  discriminator so the rasterized HUD is reused across the steady-state
  span of the timeline.
• mp4 export example: examples/demo_timeline_mp4.rs reuses the same
  demo_scene module via #[path] so the live preview and the offline
  encoder always render the same scene.