Near-fullscreen overlay + cursor + sub-rect raycast for 2D-surround (#131)

[dfattal]

Fullscreen 2D-surround region-editor support for the transparent overlay (#131), with a clean, deterministic startup.

Overlay geometry (native, Win32)

• displayxr_set_fullscreen_overlay_pref (new, opt-in): when set before session create, the transparent overlay is born covering nearly the whole monitor — the monitor rect minus 1px on right+bottom — instead of Unity's small windowed client rect.
  • The 1px keeps the window DWM-composited. An exact-monitor window trips Windows fullscreen-optimization / independent-flip (DWM-alpha bypass), which was the source of the startup white flashes + hang. Confirmed against a clean-era log where a near-full 3517×2160 overlay was perfectly smooth — coverage was never the problem, exact-monitor was.
  • Still covers the taskbar (tiger feet render over it). Born at final size → never resized (a resize recreates the presentation swapchain = a flash). The transparent-enable snap and set_overlay_fullscreen are no-ops when already near-fullscreen.
• displayxr_set_overlay_cursor (new): app-driven overlay cursor shape (arrow / resize / move) via WM_SETCURSOR for the region editor.

Sub-rect-aware interaction (C#)

• displayxr_get_canvas_rect_px binding + GetStereoViewport so the cyclopean raycast in DisplayXRTransparentOverlay maps the cursor through the canvas sub-rect (matching the sub-rect Kooima projection from Sub-rect-aware Kooima: physically-correct 3D inside the surround canvas (#131) #135) — fixes LMB-rotate when a canvas sub-rect is active. Zero change with no sub-rect.
• DisplayXRTransparentOverlay.RequestFullscreenOverlay wrapper.

Diagnostics

• Coarse [+ms] relative timestamps on every native log line.

Windows-only native (win32.c); macOS resolves via C# EntryPointNotFound fallback. Hardware-validated on RTX 3080 + Leia SR: clean, deterministic startup across launches.