Detect and surface host device loss instead of spinning silently

D3D11Window ignored the HRESULTs from Present, ResizeBuffers, and
CreateRenderTargetView, so a host-side TDR / driver reset / GPU hang left the
render loop presenting to a dead device forever with no error.

Now note_device_loss() inspects those HRESULTs; on DXGI_ERROR_DEVICE_REMOVED/RESET
it captures GetDeviceRemovedReason() and sets device_lost(). The main loop checks
it after render_frame, shows a MessageBox with the reason, and stops cleanly.

Per the agreed scope this is detect-surface-halt, not full device re-creation
(which would have to re-init ImGui + the capture pipeline) -- that's future work.
Not unit-testable (TDR isn't deterministically reproducible); fixed by inspection.

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

README.md

@@ -114,9 +114,6 @@ From an in-depth review pass. Each item is fixed test-first (a failing test, the
 as its own commit; "verify" items are confirmed real before any change, and dropped if not.
 
 Confirmed bugs:
-- **Host device loss unhandled** — `Present`/`ResizeBuffers`/`CreateRenderTargetView` HRESULTs are
-  ignored and a removed/reset device spins silently. Detect `DEVICE_REMOVED/RESET`, log
-  `GetDeviceRemovedReason`, surface it, and halt the render loop cleanly.
 - **Keyed-mutex `WAIT_ABANDONED` not handled** — `shared_texture` treats it as "skip", so a mirror
   never recovers after a host crash + reconnect. Treat it as acquired (copy + release).