feat(cua-driver): add multi-display cursor support for macos

[injaneity]

summary

• make negative macos cursor coordinates valid
• create one cursor surface per connected display
• render each surface with its display origin and backing scale
• rebuild surfaces after display changes and reject stale frames

implementation

macos keeps one shared cursor state and render worker, while appkit owns one transparent surface per display. cursor artwork is rendered into every display it intersects, so seam crossings do not clip the pointer. only displays with current or previous pixels are rendered, which also clears the old display after movement.

placement is explicit instead of encoded as a coordinate. theme-derived paint bounds select intersecting displays without rendering every display for custom themes. the session badge remains on the cursor's anchor display, and target-relative z-order applies to every surface carrying cursor pixels.

validation

• cargo test -p cursor-overlay --locked — 43 passed
• cargo test -p platform-macos --lib --locked — 350 passed, 1 environment-mutating test ignored
• cargo test -p platform-linux --lib --locked — 16 host-available tests passed
• cargo test -p platform-windows --lib --locked — 42 host-available tests passed
• cross-platform cargo checks passed
• rustfmt and diff checks passed

gap before readiness

the candidate still needs live macos validation with left, upper, mixed-scale, cross-display, and hotplug layouts. the pr remains draft until that evidence exists.

refs #2992