Canvas multi-image visibility now broadcasts a signal extensions can
subscribe to. Pure-additive; existing single-image canvas consumers
see no change.

  - setImageVisible(id, visible) now emits `image-visibility-change`
    on the handle bus when the hidden set actually changes (no-op
    re-toggles stay silent). Payload: {imageId, visible}. Lets
    extensions pinned to a specific image — Etcher annotations,
    ML overlays, comment threads — hide / restore in lockstep with
    the host's `display: none` on the image's <img>.
  - handle.getHiddenImageIds() returns a snapshot of the
    currently-hidden image ids as a plain array. Seeds late-
    mounting extensions; the event isn't replayed.
  - setImageVisible is now on the canvas handle's public surface
    (was previously only on the underlying controller).

Etcher 0.4.7 ships per-image shape tagging on multi-image canvases
(paged manga / spread readers / lookbooks) so it can hide shapes
whose host image is currently display:none. The host already calls
setImageVisible; this release adds the signal Etcher needs to
mirror that state.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>