Edit compatible glyph sources together

[kostyafarber]

Problem

Glyph editing currently targets one authored source layer at a time. Designers cannot select compatible sources together, compare them automatically, or apply one atomic edit across their corresponding contours and points. Per-layer entity IDs also differ, so frontend code needs a Rust-owned structural match rather than assuming IDs or rebuilding topology rules.

Expected outcome

A designer can keep one source as the editing reference, select additional compatible sources, see those sources as outline overlays, and apply supported glyph edits to every selected source as one undoable operation.

Acceptance criteria

• Source rows support single selection, contiguous range selection, and noncontiguous toggle selection while retaining one reference source.
• Selecting all sources and collapsing back to the reference are available from keyboard commands.
• Other selected sources appear as live outline overlays without replacing the reference source.
• Rust derives deterministic contour, point, anchor, and component matches between compatible authored layers while preserving per-layer IDs.
• Move, resize, rotate, point insertion, point deletion, and Pen edits use those matches across selected compatible layers.
• A multi-source edit is atomic and creates one undo entry.
• Incompatible layers are not partially edited and expose structural diagnostic evidence.

Related to #98 and #383.