feat(host): HDR Vulkan layer so Vulkan games get HDR on the virtual display

NVIDIA/AMD Vulkan ICDs refuse to *advertise* an HDR color space for a surface on an
IddCx indirect/virtual display, so Vulkan games (Doom: The Dark Ages, id Tech, Indiana
Jones, …) report "device does not support HDR" — even though Windows HDR, DWM compose,
and the client PQ stream all work, and the ICD happily *accepts + presents* a forced HDR
swapchain there. The whole gap is enumeration; the community (Apollo/Sunshine/VDD) wrote
this off as kernel-side / unfixable.

Add VK_LAYER_PUNKTFUNK_hdr_inject (packaging/windows/pf-vkhdr-layer/): a standalone
cdylib Vulkan implicit layer that appends {A2B10G10R10, HDR10_ST2084} + {RGBA16F, scRGB}
to vkGetPhysicalDeviceSurfaceFormats[2]KHR (no need to hook vkCreateSwapchainKHR — the
ICD doesn't validate the color space there). Self-gated on the surface monitor's actual
advanced-color state (DisplayConfig GET_ADVANCED_COLOR_INFO), so it is a complete no-op
on SDR sessions and real monitors (dedup). Always-on (registry-discovered) so it works
regardless of how a game is launched — env-scoping silently fails for already-running
Steam. Escape hatches: DISABLE_PF_VKHDR, PF_VKHDR_EXCLUDE, and a built-in kernel-anti-
cheat denylist.

The installer builds/signs/stages it and registers it under
HKLM64\SOFTWARE\Khronos\Vulkan\ImplicitLayers (opt-out "Install the HDR Vulkan layer"
task); windows-host CI fmt+clippy-gates it (msvc-only FFI).

Live-validated on the RTX box: Doom: The Dark Ages enables HDR over the pf-vdisplay
virtual display.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/windows/punktfunk-host.iss

@@ -56,6 +56,13 @@
     #define WithWeb
   #endif
 #endif
+; VkLayerDir (the staged pf-vkhdr-layer: pf_vkhdr_layer.dll + .json) is optional — present when the
+; HDR Vulkan layer was built. It lets Vulkan games (Doom: The Dark Ages, etc.) enable HDR over the
+; virtual display (the ICD won't advertise HDR there; the layer injects the surface formats, self-
+; gated on the display's actual HDR state).
+#ifdef VkLayerDir
+  #define WithVkLayer
+#endif
 
 [Setup]
 AppId={{7C9E6A52-1F4B-4E8D-A3C7-2B5D8F1E0A93}
@@ -89,6 +96,9 @@ Name: "installdriver"; Description: "Install the pf-vdisplay virtual display dri
 #ifdef WithGamepad
 Name: "installgamepad"; Description: "Install the virtual gamepad drivers (DualSense / DualShock 4 / Xbox 360 — no ViGEmBus needed)"
 #endif
+#ifdef WithVkLayer
+Name: "installhdrlayer"; Description: "Install the HDR Vulkan layer (lets Vulkan games like Doom use HDR on the virtual display)"
+#endif
 Name: "startservice"; Description: "Start the punktfunk host service now (also starts on every boot)"
 
 [Files]
@@ -119,6 +129,22 @@ Source: "{#StageDir}\*"; DestDir: "{tmp}\pfvdisplay"; Flags: deleteafterinstall
 ; The vendored UMDF gamepad drivers + install-gamepad-drivers.ps1, extracted to {tmp}, removed after.
 Source: "{#GamepadStageDir}\*"; DestDir: "{tmp}\gamepad"; Flags: deleteafterinstall recursesubdirs createallsubdirs; Tasks: installgamepad
 #endif
+#ifdef WithVkLayer
+; The HDR Vulkan implicit layer (cdylib + its JSON manifest) laid into {app}\vklayer and registered
+; below. The manifest's library_path is ".\pf_vkhdr_layer.dll" (relative to the JSON), so the two
+; must live in the same directory.
+Source: "{#VkLayerDir}\pf_vkhdr_layer.dll"; DestDir: "{app}\vklayer"; Flags: ignoreversion; Tasks: installhdrlayer
+Source: "{#VkLayerDir}\pf_vkhdr_layer.json"; DestDir: "{app}\vklayer"; Flags: ignoreversion; Tasks: installhdrlayer
+#endif
+
+[Registry]
+#ifdef WithVkLayer
+; Register the HDR Vulkan implicit layer system-wide. The 64-bit Vulkan loader reads
+; HKLM64\SOFTWARE\Khronos\Vulkan\ImplicitLayers; the value NAME is the manifest path and the DWORD
+; DATA is 0 (= enabled). uninsdeletevalue removes just this value on uninstall. The layer is inert
+; unless the target display has HDR enabled, and honors DISABLE_PF_VKHDR=1 as a global off-switch.
+Root: HKLM64; Subkey: "SOFTWARE\Khronos\Vulkan\ImplicitLayers"; ValueType: dword; ValueName: "{app}\vklayer\pf_vkhdr_layer.json"; ValueData: 0; Flags: uninsdeletevalue; Tasks: installhdrlayer
+#endif
 
 [Run]
 #ifdef WithDriver