refactor(test-apps): migrate Windows cube_handle capture to xrCaptureAtlasEXT [#396 W6]

[dfattal]

Summary

W6 consumer migration (test apps) of #396. All four Windows cube_handle_* apps now capture the atlas via the runtime (XR_EXT_atlas_capture) instead of per-API GPU readbacks, behind one shared helper — net −350 lines.

Refactor (test_apps/common)

• New dxr_capture::RequestRuntimeAtlasCapture(xr, appName, cols, rows, hwnd) in atlas_capture.cpp (sr_common_base): the single graphics-API-agnostic capture path — 3D-mode guard + MakeCaptureAtlasPrefix numbering + xrCaptureAtlasEXT (PROJECTION_ONLY) + flash + logging. Each app's ~30-line I-key block → one call.

Migration (d3d11 / d3d12 / gl / vk _win)

• main.cpp: capture block → RequestRuntimeAtlasCapture(); dropped the per-API CaptureAtlasRegion* fallback (test apps always run against this repo's runtime, which exports the ext).
• xr_session.cpp (d3d12/gl/vk): detect + enable + resolve XR_EXT_atlas_capture, mirroring d3d11.

Dead-code removal

• Deleted atlas_capture_d3d11.cpp + atlas_capture_d3d12.cpp (Windows-only, now unused) and their declarations; removed the D3D11 readback from sr_common and the per-API TUs from the d3d12/gl/vk app CMakeLists.
• Kept atlas_capture_{gl,vk}.cpp + atlas_capture_metal.mm — the macOS cube_handle apps still use them and are not migrated here (can't build/verify on Windows; follow-up on a Mac).

Verification (in-process, real display)

• d3d11 (helper refactor) and d3d12 (new PFN resolve): I → a tight 1280×360 two-view PNG via xrCaptureAtlasEXT (xrCaptureAtlasEXT: resolved + Atlas capture requested).
• gl / vk: build + link with the identical pattern (capture path is the shared helper).

Follow-ups

• macOS cube_handle_{metal,gl,vk}_macos migration (needs a Mac) — then atlas_capture_{gl,vk}.cpp + atlas_capture_metal.mm can go too.
• Demos (gauss, modelviewer) + Unity/Unreal consumer migration.

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