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.

debian/changelog

@@ -1,5 +1,17 @@
 g15stats (3.0.3+git20240609.95de11a-1~local1) UNRELEASED; urgency=medium
 
+  * Add cached multi-vendor GPU summary columns: independent read-only amdgpu
+    workers and optional, deadline-bounded batched shell-free nvidia-smi
+    queries.
+    Collect all supported cards by default, display two columns concurrently
+    beside compact CPU/RAM data, and cycle larger inventories in sorted pairs.
+    Document PCI selection, bounds, units, missing metrics and unchanged no-GPU
+    layouts; add multi-adapter isolation, live collector and framebuffer tests.
+    Check complete GPU/TOTAL/footer glyphs against font bits independently;
+    exercise off/single/paged layouts and verify lossless visual exports.
+  * Initialize the bitmap canvas locally, avoiding the optional FreeType
+    canvas ABI mismatch that corrupts the heap during application startup.
+    Add a full-main regression with isolated daemon transport.
   * Import Debian 1.9.2-2 packaging and manual, retaining its changelog,
     attribution and checksum-recorded archive provenance.
   * Package Menelkir master 95de11a526346b3d9fd33df051b330c36b1c8a16,