Make Pen visual snapshots layout-independent

[kostyafarber]

Problem

Pen visual tests mix locator-relative clicks with absolute page mouse coordinates and fixed delays. Layout changes can therefore draw different geometry while thin-stroke snapshot diffs still appear numerically small, making local and merge-queue results unreliable.

Expected outcome

Pen snapshots use deterministic canvas-relative gestures, prove the authored geometry before comparing pixels, and capture only stable editor canvas surfaces.

Acceptance criteria

• Raw Pen mouse gestures derive page coordinates from normalized positions inside #interactive-canvas.
• Snapshot assertions wait for observable editor state and stable locator screenshots instead of fixed delays.
• Thin geometry snapshots remain exact rather than relying on broad pixel-ratio tolerances.
• The focused Pen visual suite passes repeatedly with reviewed baselines.
• The E2E guide records the snapshot-writing rules for future developers and agents.