Add numeric label scrubbing and interaction coverage

[Fe4rlessCloak]

Summary

• Drag numeric labels horizontally to adjust their adjacent values. Numeric fields still accept typed input when clicked.
• Apply scrubbing across New Canvas, tool controls, layer opacity, and sizing and adjustment sheets, using each control’s existing limits.
• Let the empty canvas leave cursor control to the New Canvas form, while preserving tool cursors when a document is open.
• Add UI tests for dragging, limits, typing, opacity undo, Canvas Size, and Image Size. Run UI tests serially in CI.

Testing

• Full local Xcode suite after rebasing onto main: 436 tests passed, 0 failed.
• Cursor callbacks are covered by tests. Pointer appearance on hover, leave, and release outside a label should be checked with the actual macOS pointer.

Known pre-existing issue

In Image Size, choose Inches with Resample enabled, enter an invalid negative Resolution such as -5, then correct it to 72 without closing the sheet. The pixel dimensions can grow unexpectedly. This also occurs in the v1.2.10 release. This PR prevents scrubbing the dimension labels while the physical resolution is invalid; it does not change the existing resolution-recovery behavior.

[robbietilton]

Merged, thanks — label scrubbing feels great. A few changes on main:

• Dragging snaps to whole numbers in Canvas Size, Image Size, the Type bar (size, tracking, leading) and the transform bar (X, Y, W, H, scale, angle). Typing still accepts decimals.
• New Canvas width and height aren't draggable.
• Left the UI tests out: I'm keeping CI to the unit tests, so the test plan, scheme and workflow changes aren't included. Your unit test updates are in.