feat: drop the dxcompiler xenia-edge no longer loads

The D3D12 backend used to load dxcompiler.dll and dxil.dll and refused
to start without the compiler. It now compiles shaders through Mesa and
keeps only the signer: D3D12 rejects an unsigned shader, so
PipelineCache::Initialize stops on IsSignerAvailable() and asks for a
recent dxil.dll. dxcompiler appears nowhere in the tree any more, and
the installer extracts that one entry from the same pinned archive.

The profile still declared dxcompiler.dll required, so it sent people
after a file the emulator stopped reading. The gpu cvar moved to
emulator.cc beside the code that reads it.

emulators/xenia-edge.yml

@@ -74,36 +74,28 @@ notes: |
 files:
   # -- Direct3D 12 runtime, Windows only --
 
-  - name: dxcompiler.dll
-    path: "D3D12/dxcompiler.dll"
-    system: microsoft-xbox-360
-    required: true
-    description: "DirectX Shader Compiler 1.9.2602.24, DXIL compilation for the Direct3D 12 backend"
-    source_ref: "src/xenia/ui/d3d12/d3d12_provider.cc:279-322, src/xenia/gpu/d3d12/pipeline_cache.cc:121-134, src/xenia/ui/redist_installer_wx.cc:79-85, 119-122, 594-629"
-    note: >-
-      Loaded by full path from the D3D12 folder next to the executable, then by
-      plain name from the system search path. PipelineCache::Initialize returns
-      false without it because guest and system shaders are all DXIL, and that
-      failure propagates to "Failed to setup graphics_system!"
-      (emulator.cc:430-435). Windows defaults to gpu = "d3d12"
-      (src/xenia/app/xenia_main.cc:70) and the backend factory only falls back when
-      D3D12GraphicsSystem::IsAvailable() is false, which tests the D3D12 API
-      itself, not the redistributable. Taken from bin/x64/dxcompiler.dll in
-      dxc_2026_05_27.zip of DirectXShaderCompiler release v1.9.2602.24, pinned
-      at sha256 cf658aacf070d3045e31b8f1f8a696c2945f37c1095019481ef7c513368db3b4;
-      bin/arm64 on ARM64 builds.
-
   - name: dxil.dll
     path: "D3D12/dxil.dll"
     system: microsoft-xbox-360
     required: true
-    description: "DXIL signing and validation library shipped with DirectX Shader Compiler 1.9.2602.24"
-    source_ref: "src/xenia/ui/d3d12/d3d12_provider.cc:286-292, src/xenia/gpu/d3d12/spirv_to_dxil_compiler.cc:154-169, 194-200, src/xenia/ui/redist_installer_wx.cc:79-85, 119-122"
+    description: "DXIL signer and validator from DirectX Shader Compiler 1.9.2602.24"
+    source_ref: "src/xenia/ui/d3d12/d3d12_provider.cc:236-260, src/xenia/gpu/d3d12/pipeline_cache.cc:113-121, src/xenia/ui/redist_installer_wx.cc:79-86, 120-122, 174-186, 588-621"
     note: >-
-      Preloaded by full path so dxcompiler's own plain-name load resolves to it
-      and the DXIL leaves signed; spirv_to_dxil queries its validator version to
-      stamp the container the validator will accept. Same archive and pin as
-      dxcompiler.dll, from bin/x64 or bin/arm64.
+      The only shader library the backend still needs. Loaded by full path from
+      the D3D12 folder next to the executable, then by plain name from the
+      system search path, because the signer's own plain-name load skips
+      D3D12/. D3D12 rejects an unsigned shader, so PipelineCache::Initialize
+      stops when IsSignerAvailable() is false and asks for a recent dxil.dll in
+      the D3D12 folder; that failure propagates to "Failed to setup
+      graphics_system!" (src/xenia/emulator.cc:427-431). Windows defaults to
+      gpu = "d3d12" (src/xenia/app/xenia_main.cc:70) and the backend factory
+      only falls back when D3D12GraphicsSystem::IsAvailable() is false, which
+      tests the D3D12 API itself, not the redistributable. Taken from
+      bin/x64/dxil.dll in dxc_2026_05_27.zip of DirectXShaderCompiler release
+      v1.9.2602.24, pinned at sha256
+      cf658aacf070d3045e31b8f1f8a696c2945f37c1095019481ef7c513368db3b4;
+      bin/arm64 on ARM64 builds. It is the single entry the installer extracts
+      from that archive.
 
   - name: D3D12Core.dll
     path: "D3D12/D3D12Core.dll"