Make the host per-monitor DPI aware and scale the ImGui overlay

The window sized itself to GetSystemMetrics(SM_CXSCREEN/CYSCREEN) but the
process was DPI-unaware, so on a scaled display (e.g. 4K @ 150%) Windows
handed us a virtualized resolution and bitmap-stretched the whole window up
to native -- softening the mirror, which is the tool's entire point.

Declare per-monitor-v2 awareness at startup so GetSystemMetrics/GetDpiForWindow
report true pixels. That alone would shrink the fixed-13px ImGui overlay to
crisp-but-tiny, so pair it with UI scaling: rebuild the default-font atlas at a
DPI-scaled SizePixels (crisp at the target size, unlike FontGlobalScale's
bitmap stretch) and ScaleAllSizes() the style. Net: same physical size as
before, now sharp.

- common/include/coop/dpi.hpp: pure DPI->scale math (uses USER_DEFAULT_SCREEN_DPI
  and a named kBaseFontPx, not bare 96/13 literals), with a zero fallback and
  clamping. Unit-tested by tests/dpi_test.cpp.
- imgui_layer: apply_dpi() at init from GetDpiForWindow; set_dpi() for runtime
  changes (rebuild atlas + reset/scale style + invalidate the DX11 font texture).
- d3d11_window: latch WM_DPICHANGED (honor the suggested rect), expose
  take_dpi_change(); main loop polls it and calls imgui.set_dpi() between frames.

The DPI math is unit-tested; the actual awareness + font rasterization + live
WM_DPICHANGED rescale are verified by hand.

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

host/src/d3d11_window.cpp

@@ -334,6 +334,22 @@ LRESULT CALLBACK D3D11Window::wnd_proc(HWND hwnd, UINT msg, WPARAM wparam, LPARA
 			self->resize_height_ = HIWORD(lparam);
 		}
 		return 0;
+	case WM_DPICHANGED:
+		// Per-monitor-v2: the DPI of the display we're on changed. Resize to the rect Windows suggests
+		// in lparam (its recommended handling; the resulting WM_SIZE repaints the swap chain via the
+		// deferred-resize path above), then latch the new DPI for the overlay to rescale its font/style.
+		if (self != nullptr)
+		{
+			if (const auto* suggested = reinterpret_cast<const RECT*>(lparam); suggested != nullptr)
+			{
+				SetWindowPos(hwnd, nullptr, suggested->left, suggested->top,
+				             suggested->right - suggested->left, suggested->bottom - suggested->top,
+				             SWP_NOZORDER | SWP_NOACTIVATE);
+			}
+			self->dpi_pending_ = true;
+			self->pending_dpi_ = HIWORD(wparam); // X and Y DPI are equal; HIWORD is the Y value
+		}
+		return 0;
 	case WM_SYSKEYDOWN:
 		// F10 is our screenshot key; ImGui already saw this message (handler runs above),
 		// so swallow it here to stop DefWindowProc from flicking into Win32 menu mode.