Expose Windows onboarding to UI Automation

[coneilen]

Summary

Exposes the four-page custom Windows onboarding modal as a named UI Automation tree with Invoke-capable actions and deterministic keyboard focus. This is layer 2 above #579 and preserves the early daemon connection, modal ownership, foregrounding, teardown, navigation, and backend selection behavior.

Part of #556.

Changes

• Add an onboarding-specific IRawElementProviderFragmentRoot for the current page, Skip, Back, Continue/Get Started, and backend actions.
• Route WM_GETOBJECT, UIA Invoke/SetFocus, mouse input, and Tab/Shift+Tab/Enter/Space through one deterministic onboarding action model with visible focus outlines.
• Keep the shared shell provider unchanged apart from a two-line include that compiles the isolated onboarding provider.

Test plan

RED: zig test src\WindowsOnboarding.zig -target x86_64-windows-msvc -lc -luser32 -lgdi32 -> 11/12 passed; deterministic Tab traversal failed with TestUnexpectedResult
GREEN: zig test src\WindowsOnboarding.zig -target x86_64-windows-msvc -lc -luser32 -lgdi32 -> 13/13 passed
REGRESSION: zig test src\App.zig src\AccessibilityProvider.cpp src\FilePicker.c winghostty-win32-host.lib and ghostty-vt-static.lib with Windows system libraries -> 734/734 passed

Additional provider RED: zig test src\WindowsOnboarding.zig ... --test-filter "onboarding routes WM_GETOBJECT" -> 0/1 passed; the assertion found no WM_GETOBJECT route.

Additional verification:

• zig test src\WindowsOnboarding.zig src\AccessibilityProvider.cpp ... -> 13/13 passed with the native provider compiled and linked.
• zig build -Doptimize=ReleaseSafe -Dwinghostty-dir=... -Dwinghostty-lib=... -> passed.
• Isolated shown-window probe with owned GRAPHCODE_SUPPORT_DIR, LOCALAPPDATA, TEMP, TMP, and USERPROFILE roots -> named root and four pages discovered; Skip, Back, Continue/Get Started, and Copilot CLI exposed Invoke; backend selection reported Selected; UIA SetFocus reached Get Started; posted native WM_KEYDOWN Tab traversed Continue -> Skip -> Continue.
• Desktop-level injected keystrokes were not observed by this host, so this does not claim physical keyboard injection, packaged install, screen-reader conformance, or package install -> first launch -> connected Welcome coverage.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure