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-layouts.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Native-pixel checks only: no image conversion, daemon or input-device access.
+set -eu
+renderer=${RENDER_TEST:-./render-test}
+out=$(mktemp -d "${TMPDIR:-/tmp}/g15stats-layouts-XXXXXX")
+trap 'rm -rf "$out"' EXIT HUP INT TERM
+"$renderer" off "$out/off"
+"$renderer" off "$out/single" 1
+"$renderer" off "$out/dual" 2
+"$renderer" off "$out/large" 2 0 large
+"$renderer" off "$out/unknown" 2 0 unknown
+"$renderer" off "$out/odd" 3 1
+"$renderer" off "$out/maximum" 64 31