[MG_Backend] (P7): an unsupported texel-buffer format reads zeros instead of killing the session

`texel-buffer-native-format@P7` becomes a decline (rule I (a)), and with it the
last refusal site in UniformManager.cpp is gone. GL's buffer-texture format list
is wider than what Vulkan mandates for texel buffers - GL_RGB32{F,I,UI} and the
16-bit UNORM members are the realistic misses on a real driver - so this is a
device capability gap, not anything the application did wrong and nothing it
could have sent differently.

The placeholder is the right answer here where it was not for the unaligned range
in 7c373d60, and the difference is whether a correct answer exists at all. There
the bytes are addressable and only the descriptor cannot name them, so a
placeholder would have hidden a fixable gap behind silently dropped writes. Here
the device cannot represent this format as a texel buffer in any form, so the
honest observable is monolith's for a missing view: the fetch reads zeros and the
draw survives. AcquireUnboundTexelBufferView already falls back to the R32 member
of the shader's numeric class, whose texel-buffer support is mandatory, so the
fallback cannot itself fail for want of device features.

red-once (R-16). lavapipe supports every format the buffer-texture cases use, so
the site is not reachable on the host - the red is the same case with the device
support check forced to fail, both shapes built and run back to back:

  DirectVulkan.{Split,Spawn}.Full.BufferTextureScenario.VertexStageTexelFetchReadsTheBufferAndTracksItsUpdates
    unforced  2/2 Passed (the format IS supported here)
    before (7c373d60 + forced)  Subprocess aborted -
                                Fatal{UnmigratedVerb, "Magma:texel-buffer-native-format@P7"}
    after  (this commit + forced)  ***Failed on the texel data and NOT on a Fatal:
                                the fetch read the zero placeholder, the session lived
    restored  2/2 Passed, no force markers left

Which format actually trips this is a device question, not a host one. Adreno is
where GL_RGB32F buffer textures are expected to miss, so the decline's real
observable wants a device pass; notes/p7/magma-a.md lists it.

Lanes: integration-magma-split 75/75, -spawn 54/54, -tcp 56/56,
integration-magma-buffers 21/21. `grep -n '@P7'` on this file now returns one
line, and it is the comment that records the A.1 retirement - no refusal sites
remain.

MobileGL/MG_Backend/DirectVulkan/Renderer/UniformManager.cpp

@@ -1153,8 +1153,27 @@ namespace MobileGL::MG_Backend::DirectVulkan {
                 state.BoundShaderImages[unit].InternalFormat));
         if (format == VK_FORMAT_UNDEFINED) format = MG_Util::ConvertTextureInternalFormatToVkEnum(internalFormat);
         const auto feature = storage ? VK_FORMAT_FEATURE_STORAGE_TEXEL_BUFFER_BIT : VK_FORMAT_FEATURE_UNIFORM_TEXEL_BUFFER_BIT;
-        if (format == VK_FORMAT_UNDEFINED || !BufferFormatSupportsFeature(m_physicalDevice, format, feature))
-            WireDescriptorFatal("texel-buffer-native-format@P7");
+        if (format == VK_FORMAT_UNDEFINED || !BufferFormatSupportsFeature(m_physicalDevice, format, feature)) {
+            // P7 A.5, a decline (rule I (a)). GL's buffer-texture format list is wider than what
+            // Vulkan mandates for texel buffers - GL_RGB32{F,I,UI} and the 16-bit UNORM members
+            // are the realistic misses on a real driver - so this is a device capability gap, not
+            // anything the application did wrong, and nothing it could have sent differently.
+            //
+            // The placeholder is the RIGHT answer here, where it was not for the unaligned range
+            // above, and the difference is whether a correct answer exists at all. There, the
+            // bytes are addressable and only the descriptor cannot name them, so a placeholder
+            // would have hidden a fixable gap behind silently dropped writes. Here the device
+            // cannot represent this format as a texel buffer in any form - so the honest
+            // observable is monolith's for a missing view: the fetch reads zeros and the draw
+            // survives. AcquireUnboundTexelBufferView falls back to the R32 member of the
+            // shader's numeric class, whose texel-buffer support is mandatory, so the fallback
+            // itself cannot fail for want of device features.
+            MGLOG_E_ONCE("ResolveWireTexelBufferDescriptor: binding %u wants VkFormat %d as a %s texel buffer and "
+                         "this device does not support it; the binding falls back to the zero placeholder and the "
+                         "fetch reads zeros (texel-buffer-native-format)",
+                         binding, static_cast<Int>(format), storage ? "storage" : "uniform");
+            return placeholder();
+        }
         const VkDeviceSize texelSize = MG_Util::GetSizedInternalFormatSizeInBytes(internalFormat);
         VkDeviceSize start = 0, size = 0;
         if (texelSize == 0 || !ResolveWireRange(record.Desc.BufOffset, record.Desc.BufSize, slice.size, start, size)) return false;