Added foveated rendering for the Quest

- Eyes render under a VK_EXT_fragment_density_map: full rate around each eye's forward direction,
  2x2 then 4x4 pixel blocks towards the edges ([vr] foveation = off|low|medium|high, default off).
  XR_FB_foveation cannot help here: the runtime's maps only shape passes drawing into its
  swapchain, and the eyes reach it through a copy.
- aurora-main/patches/dawn/aurora_fdm.inc: Dawn enables the extension only on request and for
  dynamic rendering, flags every render pipeline, and chains an immutable RG8 map into any pass
  whose first color attachment is a view bound to one (ABI: include/aurora/dawn_fdm_abi.h).
- android/Build-QuestDawn.ps1 builds the pinned Dawn revision with those patches for arm64
  (dawn-build CI flags, protobuf off) into a cached package; Build-Quest.ps1 links it
  (-StockDawn opts out) and AuroraDawnProvider.cmake enables the ABI from its manifest.
- lib/gfx/foveation.hpp generates the maps (32 px per texel, densities 255/127/63); an eye is
  foveated only when single_pass_eyes draws it in one render pass. Menus never are.
- Live level from the headset panel's VR tab and the launcher; the launch decides whether the
  device has maps. debug.wiicompiled.foveation and debug.wiicompiled.fdm for A/B.
- Tests: Foveation cases in gx_fifo_tests, mkw_vr_config_tests. Docs: OPENXR.md, quest-port.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/Build-Quest.ps1

@@ -15,13 +15,19 @@
 # Prerequisites (see docs/quest-port.md): JDK 17, the Android SDK with NDK
 # 29.0.14206865 and CMake 3.22.1, and android/Prepare-QuestDependencies.ps1
 # having staged the SDL3 AAR.
+#
+# Dawn is built from source with Aurora's patches (android/Build-QuestDawn.ps1:
+# fragment density maps for foveated rendering) and cached under
+# .scratch/quest-dawn; the first build takes a while. -StockDawn links the
+# stock prebuilt package instead, which leaves foveated rendering unavailable.
 [CmdletBinding()]
 param(
     [string]$Generated = '',
     [string]$Dependencies = '',
     [string]$CMakeDir = '',
     [ValidateSet('debug', 'release')] [string]$Configuration = 'debug',
-    [switch]$Install
+    [switch]$Install,
+    [switch]$StockDawn
 )
 $ErrorActionPreference = 'Stop'
 
@@ -79,6 +85,11 @@ $gradleArgs = @(
     "-PmkwGeneratedDir=$Generated"
 )
 if ($Dependencies) { $gradleArgs += "-PmkwDependenciesDir=$Dependencies" }
+if (-not $StockDawn) {
+    $dawnWork = Join-Path $repo '.scratch\quest-dawn'
+    & (Join-Path $root 'Build-QuestDawn.ps1') -WorkDirectory $dawnWork
+    $gradleArgs += "-PmkwQuestDawnDir=$(Join-Path $dawnWork 'package')"
+}
 $gradleArgs += $task
 Write-Host "gradlew $($gradleArgs -join ' ')"
 & (Join-Path $root 'gradlew.bat') @gradleArgs