Add mixed-vendor GPU summary and fix startup heap corruption

Collect AMD amdgpu sysfs and optional NVIDIA nvidia-smi stats in the
background, auto-detecting supported adapters in PCI order and rotating
additional pairs alongside CPU/RAM. Explicit PCI selection and off mode
remain supported; missing metrics render as N/A.

Initialize the bitmap-only canvas locally instead of the ABI-dependent
libg15render initializer: the installed TTF-enabled renderer wrote an
8-byte FreeType pointer past the smaller bitmap-only allocation, corrupting
the heap before GPU threads started. Add collector, rendering and full-main
regression tests.

tests/render-images.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+# Convert existing render-test exports without trimming, resampling or labels
+# inside the framebuffer. ImageMagick is optional, not a make check dependency.
+set -eu
+if [ "$#" -ne 1 ]; then
+    echo "Usage: sh tests/render-images.sh ABSOLUTE_OUTPUT_DIRECTORY" >&2
+    exit 2
+fi
+cd "$1"
+for pgm in ./*-[01].pgm; do
+    [ -f "$pgm" ] || continue
+    base=${pgm%.pgm}
+    [ "$(wc -c < "$base.bin")" -eq 860 ]
+    { printf 'P4\n160 43\n'; cat "$base.bin"; } > "$base.pbm"
+    # Compare the packed 1-bit buffer independently with the PGM export.
+    compare -metric AE "$base.pbm" "$pgm" null: 2> "$base-pixels.log"
+    convert "$pgm" +repage -type TrueColor "PNG24:$base-native.png"
+    convert "$base-native.png" -sample 800% +repage "PNG24:$base-8x.png"
+    convert "$base-8x.png" -sample 160x43! +repage "PNG24:$base-roundtrip.png"
+    compare -metric AE "$pgm" "$base-native.png" null: 2>> "$base-pixels.log"
+    compare -metric AE "$pgm" "$base-roundtrip.png" null: 2>> "$base-pixels.log"
+    rm "$base-roundtrip.png"
+    # Smaller contact tiles avoid shrinking wide sheets in image viewers.
+    # Keep the eightfold originals available for individual pixel inspection.
+    convert "$base-native.png" -sample 400% +repage "PNG24:$base-4x.png"
+done
+for group in off nvidia amd single dual large unknown odd maximum; do
+    [ -f "$group-0-4x.png" ] || continue
+    montage -font DejaVu-Sans -pointsize 14 -background '#dddddd' \
+        -label "$group: five-row summary (4x)" "$group-0-4x.png" \
+        -label "$group: four-row summary (4x)" "$group-1-4x.png" \
+        -tile 1x2 -geometry +8+8 "PNG24:contact-$group.png"
+    height=$(identify -format '%h' "contact-$group.png")
+    for variant in 0 1; do
+        top=$((8 + variant * height / 2))
+        convert "contact-$group.png" -crop "640x172+8+$top" +repage \
+            "PNG24:$group-tile-check.png"
+        compare -metric AE "$group-$variant-4x.png" "$group-tile-check.png" \
+            null: 2>> "$group-contact-pixels.log"
+    done
+    rm "$group-tile-check.png"
+done
+if [ -f contact-off.png ] && [ -f contact-nvidia.png ] && [ -f contact-amd.png ]; then
+    convert contact-off.png contact-nvidia.png contact-amd.png -append +repage \
+        PNG24:contact-legacy.png
+fi
+if [ -f contact-dual.png ] && [ -f contact-large.png ] && [ -f contact-unknown.png ]; then
+    convert contact-dual.png contact-large.png contact-unknown.png -append +repage \
+        PNG24:contact-metrics.png
+fi
+if [ -f contact-odd.png ] && [ -f contact-maximum.png ]; then
+    convert contact-odd.png contact-maximum.png -append +repage PNG24:contact-paging.png
+fi