feat(input): add native X11 xdotool pointer clicks

[UniqueDing]

Summary

• Prefer xdotool/XTEST for coordinate clicks in explicit native X11 sessions.
• Preserve the existing pointer fallback paths if xdotool is unavailable or a command fails.
• Report native-X11 xdotool availability in diagnostics/readiness.

Closes #64

[gemini-code-assist]

Caution

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[avifenesh]

@UniqueDing, thank you for the careful DDE/X11 report, the real dual-display validation, and for implementing this.

I also owe you an apology: while your branch was in flight, I merged #69 and #72 through the same input and diagnostics code without first reconciling #70. That created these conflicts and independently replaced much of your diagnostics/process work. That is on me, not on your contribution.

The core #64 fix is still missing from main: native-X11 coordinate clicks still do not use xdotool. I would like to keep this PR and merge that work after a focused rebase onto 4b4126c.

Please update it as follows:

• Drop the behavioral src/diagnostics.rs diff; main now has the broader readiness, ordering, and override handling.
• Place native-X11 xdotool clicking after the absolute-pointer attempt and before ydotool.
• Require explicit XDG_SESSION_TYPE=x11, nonempty DISPLAY, and no active WAYLAND_DISPLAY; honor COMPUTER_USE_LINUX_FORCE_YDOTOOL_POINTER.
• Use one supervised command: xdotool mousemove --sync -- X Y click --repeat N BUTTON.
• Fall back only if xdotool cannot launch. If it starts and fails, return the error rather than risking duplicate clicks.
• Resolve button support before moving, preserving correct fallback for extended buttons.
• Add focused backend/failure tests plus the short README and changelog updates discussed in #64.

If you would rather not handle the conflict resolution caused by my overlapping changes, I can update your branch while preserving your authorship and credit. Your original diagnosis and native-X11 backend are still the valuable part of this fix. Thank you again.