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.

README.rst

@@ -30,6 +30,40 @@ Requirements
 
 - libgtop
 
+GPU monitoring
+--------------
+
+The summary automatically collects ALL supported PCI addresses (amdgpu or nvidia),
+including mixed vendors. ``--gpu dddd:bb:ss.f`` (alias ``-gpu``) selects
+a card; ``--gpu off`` disables collection and restores the original layout.
+``--gpu auto`` is the default. Intel, nouveau and radeon are not supported.
+Missing metrics show N/A. No supported GPU in auto mode leaves the layout intact.
+An explicitly selected missing card keeps an N/A column. Selection is at startup.
+
+Each column shows vendor/index and PCI bus:slot.function, UTL %, U used VRAM, T total VRAM, and TEM in C on five
+lines. VRAM uses explicit MiB/GiB units (TiB/PiB if needed), with one decimal.
+One GPU retains summary labels and shortened graphs. Two GPUs appear together
+beside compact CPU/RAM-used/RAM-total/SWP rows (plus GPU count in five-row mode).
+RAM also has explicit units. More than two cycle in PCI-sorted pairs every five
+seconds from startup; all continue collecting. NV1/AMD2-style indices are one-based
+selected-inventory positions, mapped to full PCI addresses in startup diagnostics.
+The 64-card bound fails explicitly on overflow. The bottom bar and other screens
+are unchanged. AMD APU VRAM may mean its reserved
+carveout, not all shared system memory.
+
+AMD uses read-only PCI-device sysfs/hwmon files. NVIDIA uses the optional
+``/usr/bin/nvidia-smi`` without a shell or SDK dependency. Independent AMD workers
+and one PCI-keyed all-NVIDIA query wait two seconds between samples, with a shared
+750ms NVIDIA deadline and per-card five-second cache expiry. Shutdown has one
+one-second worker join/reap budget. Display refresh remains controlled by ``-r``. See README for
+exact layout, interfaces and subprocess lifecycle details. Run as an ordinary user.
+
+``make check`` covers GPU-off, single-card and multi-card native framebuffers.
+Final GPU, vertical TOTAL and time-footer glyphs are checked against font bitmap
+bits independently of the text renderer. See README for packed framebuffer
+exports and ``tests/render-images.sh``, which generates verified native/nearest-8x
+PNGs and narrow contact sheets without cropping the LCD area.
+
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