Shapes on a hidden image no longer leak into adjacent viewport
bands on multi-image <Fresco.canvas> hosts. Paged readers, spreads,
and lookbooks lay every page out side-by-side on one canvas and
call handle.setImageVisible(id, false) to hide non-active pages —
before this release the page imgs were hidden but their shapes
stayed visible, ghost-rendering inside the surrounding canvas-space
the host exposes via page-padding bands.

  - New `image_id` field on canvas-mode annotations. When a shape
    is finalized on a multi-image canvas (getImages().length > 1),
    its centroid is hit-tested against every image rect and the
    matching id is recorded on shape.image_id + emitted in
    etcher:annotations-changed. Shapes that land in empty canvas
    space get no image_id and behave like always.
  - Single-image canvas consumers are unaffected — the length > 1
    gate skips the lookup entirely.
  - Etcher subscribes to Fresco 0.5.5's `image-visibility-change`
    and toggles display:none on shapes whose image_id is in the
    hidden set (plus their title satellites). Hidden shapes that
    were being edited drop out of edit mode; hidden shapes with a
    pinned tooltip unpin — both states would otherwise float with
    no visible anchor.
  - Seeds initial state via handle.getHiddenImageIds() so an
    extension mounting after the host already toggled images off
    still picks up the current visibility.

Requires Fresco ~> 0.5.5. Older Fresco gracefully degrades — the
wiring no-ops and shapes still leak into hidden-image bands as
before.

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