Add F10 back-buffer screenshot (PNG, focus-independent)

Capture the rendered back buffer to a timestamped PNG next to the exe via
WIC, triggered by F10 (delivered even when unfocused). The capture runs in
render_frame just before Present so it includes the ImGui overlay, and reads
off the GPU so it works regardless of window focus, z-order, or occlusion. A
brief toast confirms the save (drawn the next frame, so it's never in the shot).
F10 chosen to avoid Steam's F12; its WM_SYSKEYDOWN is swallowed so Windows
doesn't enter menu mode. Documented in the Help menu + README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -81,7 +81,8 @@ and covers anything the hooked path doesn't (Vulkan, D3D9 — see Roadmap).
   assumption is visible. Streams created *after* injection are captured exactly.
 - **Debug-oriented UI:** the ImGui overlay is laid out for diagnosing the
   pipeline, not for end use. F1 hides it entirely so the window is a clean mirror
-  for RPT.
+  for RPT; F2 frees the operator cursor; **F10 saves a PNG screenshot** (back buffer,
+  written next to the exe) regardless of window focus or occlusion.
 
 ## Roadmap