windows: add native Windows support

[GGRei]

Summary

Adds native Windows support validation for gui, centered on the MSVC/vcpkg path.

This PR adds:

• native Windows setup/preflight checks with actionable diagnostics
• Windows-focused CI smoke workflow for MSVC validation
• hardened Windows native dialogs, notifications, printing delegation and D3D11 readback paths
• regression tests for the Windows bridge and setup classifier
• Windows documentation and a manual smoke checklist for behavior that cannot be fully proven in CI

Validation

• VJOBS=6 v test nativebridge/_readback_abi_test.v nativebridge/_bridge_ex_test.v
• VJOBS=6 v test _windows_setup_preflight_test.v winsetup/preflight_test.v
• v check-md README.md docs/WINDOWS.md docs/WINDOWS_MANUAL_SMOKE.md docs/PRINTING.md docs/ROADMAP.md

Notes

Some Windows behavior still requires real interactive/manual validation and is documented in docs/WINDOWS_MANUAL_SMOKE.md, including native dialogs, notification delivery, PDF handler behavior and D3D11 runtime readback/export.

[JalonSolov]

Bouncing to force pickup of CI change.

[dy-tea]

Looks fine to me.

[dy-tea]

Not sure why this was renamed?

[GGRei]

This rename is internal only.

On the Windows/MSVC path, the generated C for SpacingBundle.small hit a name/macro collision and broke _theme_bundle_test.v. Renaming the V field avoids emitting small as the C member name.

The external theme JSON format is unchanged: @[json: 'small'] keeps the JSON key as "small", so theme bundles still use "spacing": { "small": ... }.

[dy-tea]

Ah right, I forgot that windows had small defined as a macro.