examples/10-graphics: Windows stays at "builds", and the reason is a missing package

A run step was written, pushed and measured. The manifest declaration worked --
`Provisioning [xlings.workspace] entries (xim:mesa-lavapipe@26.2.0)` -- the ICD
was found in the store, and the program still printed `render unavailable`,
which is its own report that it enumerated no device.

The missing piece is the LOADER, not the driver. `compat:vulkan` ships an import
library on Windows and nothing else, and its own descriptor says why: a
statically linked loader cannot work there, because upstream's
`loader_windows.c` creates its locks in `DllMain` and a static library never
gets one. The runtime `vulkan-1.dll` is expected to come from an installed GPU
driver, and a GitHub Windows runner has none. `xim:mesa-lavapipe`'s Windows
payload is an ICD that imports system DLLs and carries no loader either.

So the example declares no device on Windows: an entry there would download
56 MB that nothing can load. macOS is not in the same position -- `compat:vulkan`
builds the loader from source everywhere except Windows -- and keeps its run
step.

Raising this platform to "runs" needs a Windows `vulkan-1.dll` package, which is
a packaging decision rather than a step in a workflow file. Both the workflow
and the example manifest record the measurement where someone looking for the
gap will find it.

Author: speak-agent

.github/workflows/ci-windows.yml

@@ -456,44 +456,28 @@ jobs:
           done
           echo "ok: both shader stages compiled and the Vulkan half linked"
 
-      # AND NOW IT RUNS. `xim:mesa-lavapipe` publishes a Windows build, so this
-      # host has a software device and the claim can be the same one Linux
-      # makes: the program reached a device and rendered the image, not merely
-      # that it compiled.
+      # WINDOWS STAYS AT "BUILDS", AND THE REASON IS A MISSING PACKAGE RATHER
+      # THAN A MISSING CI STEP.
       #
-      # The device name IS compared here, unlike on macOS. lavapipe is a
-      # software rasteriser that produces the same pixels by construction, so
-      # the image cannot distinguish it from any other implementation and the
-      # name is what does.
+      # A run step was written, pushed, and measured. The manifest declaration
+      # worked -- `Provisioning [xlings.workspace] entries
+      # (xim:mesa-lavapipe@26.2.0)` -- and the ICD was found in the store, and
+      # the program still printed `render unavailable`, which is its own report
+      # that it enumerated no device.
       #
-      # `VK_DRIVER_FILES` rather than an ICD search directory: the package is in
-      # the xlings store rather than in the registry the Windows loader reads,
-      # and naming the file is the one form that does not depend on that.
-      - name: "Graphics: the offscreen example RUNS on lavapipe"
-        shell: bash
-        run: |
-          set -e
-          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
-          # NOT installed from here, for the reason the macOS job records: the
-          # example declares `xim:mesa-lavapipe` under `cfg(windows)`, so
-          # finding the ICD without installing anything is what asserts the
-          # declaration works.
-          icd=$(find "${MCPP_HOME:-$HOME/.mcpp}/registry/data/xpkgs/xim-x-mesa-lavapipe" \
-                     "$HOME/.xlings/data/xpkgs/xim-x-mesa-lavapipe" \
-                     -name '*.json' -path '*icd.d*' -print -quit 2>/dev/null || true)
-          [ -n "$icd" ] || { echo "no lavapipe ICD in either store"; exit 1; }
-          echo "ICD: $icd"
-          cd "$GITHUB_WORKSPACE/examples/10-graphics/offscreen"
-          # `$MCPP_SELF`, which is what the build step above used. `/tmp/mcpp-fresh.exe`
-          # is a copy a LATER step makes, so naming it here would look right and
-          # fail with a missing file.
-          out=$(VK_DRIVER_FILES="$icd" "$MCPP_SELF" run 2>&1) || { echo "$out"; exit 1; }
-          echo "$out"
-          echo "$out" | grep -q 'llvmpipe' \
-            || { echo "the run did not reach the lavapipe device"; exit 1; }
-          echo "$out" | grep -qE 'centre pixel: \([0-9]+, [0-9]+, [0-9]+, 255\)' \
-            || { echo "no centre pixel was reported"; exit 1; }
-          echo "ok: lavapipe enumerated and the image was rendered on it"
+      # The missing piece is the LOADER, not the driver. `compat:vulkan` ships
+      # an import library on Windows and nothing else, and says why in its own
+      # descriptor: a statically linked loader cannot work there, because
+      # upstream's `loader_windows.c` creates its locks in `DllMain` and a
+      # static library never gets one. The runtime `vulkan-1.dll` is expected
+      # to come from an installed GPU driver, and a GitHub Windows runner has
+      # none. `xim:mesa-lavapipe`'s Windows payload is an ICD; it imports system
+      # DLLs and carries no loader either.
+      #
+      # So raising this job to "runs" needs a Windows `vulkan-1.dll` package,
+      # which is a packaging decision rather than a step in this file. macOS is
+      # not in the same position: `compat:vulkan` builds the loader from source
+      # everywhere except Windows, so that job does run.
 
       - name: "Toolchain: LLVM — build mcpp (self-host)"
         shell: bash

examples/10-graphics/offscreen/mcpp.toml

@@ -62,10 +62,18 @@ vulkan-runtime = "2026.09.07"
 [target.'cfg(macos)'.xlings.workspace]
 "xim:moltenvk" = "1.4.2"
 
-# Windows takes the same software rasteriser Linux does; upstream publishes a
-# separate build of it, one minor behind the Linux one.
-[target.'cfg(windows)'.xlings.workspace]
-"xim:mesa-lavapipe" = "26.2.0"
+# WINDOWS DECLARES NO DEVICE, AND THAT IS MEASURED RATHER THAN AN OVERSIGHT.
+#
+# `xim:mesa-lavapipe` publishes a Windows payload and it installs correctly, so
+# an entry here would work in the sense that the bytes arrive. The program still
+# reaches no device: `compat:vulkan` ships an import library on Windows and the
+# runtime `vulkan-1.dll` is expected to come from an installed GPU driver, which
+# a machine without one does not have. Declaring the driver would download 56 MB
+# that nothing can load.
+#
+# The gap is a Windows loader package. Until there is one, this platform builds
+# the Vulkan half and runs the CPU fallback, which is what the `cfg(not(...))`
+# section below carries.
 
 [build]
 accel = "vulkan1.2"