feat(computer-use): keep overlay cursor elevated when the aim point is covered

[wutongyuonce]

Summary

obscuringRects already travel from maka-cu through the observation record, and CuObservation says the agent cursor should test the point it is about to draw. keepElevated still sank whenever a window id existed, so a covered aim point could disappear under the covering window.

This PR:

• adds obscuringRects to overlay context
• copies them from the session observation record
• stays elevated only when the aim point hits a rect
• keeps today's sink behaviour when rects are missing or the point is clear

Does not change the desktop sink z-order implementation. Elevation is per point, not per whole window.

Fixes #5048

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

Verification

• npm --workspace @maka/runtime run build
• npm --workspace @maka/computer-use run build
• npm --workspace @maka/computer-use run test:dist — 122 pass, including hook-only point-in-rect cases and driveRealTool wiring (covered centre stays up, uncovered centre still sinks)
• Did not run root lint / format:check / full typecheck
• No overlay recording attached (behaviour is asserted in tests; desktop sink is unchanged)

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope:

pi-coding-agent drafted the overlay context wiring, point-in-rect keepElevated, tests, and this description. Commits carry Generated-by: pi-coding-agent.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally