F14.2b — Render every unpainted guide number at readable zoom

[Flow-Fly]

Parent: #130. Depends on #311.

Add a screen-space canvas overlay for the active guided project.

Acceptance:

• Render the stable target number for every unpainted non-transparent guide cell.
• Painted cells hide numbers; erase reveals them after history updates.
• Render numbers only at a tested readable threshold of 1600% or above.
• Below the threshold, expose a visible and DOM-accessible Zoom in to see guide numbers hint.
• Pan, zoom, undo, redo, context switching, and a 48x48 guide redraw correctly.
• Overlay does not intercept pointer input.

[Flow-Fly]

Completed in PR #315. The implementation is merged into develop at a9ed5f9 and post-merge CI run 29086892806 passed.