GPU boxes on the Processes tab: busy, engines, temps, and who has the card open

Every DRM device with a real GPU driver gets a box under the cpu box, laid out
like it: the busy curve, a meter for the device and one per engine, temperature,
clock, power and fan where the card has them, and the processes holding it open
with their share. The process table gains a Gpu% column and sort.

Device busy is Intel's RC6 idle residency inverted (what intel_gpu_top shows,
and readable without root) or amdgpu's gpu_busy_percent. Per-process shares are
DRM fdinfo deltas, as nvtop reads them; other users' fdinfo needs root, so a
daemon running as a user keeps one root child (hamlab drm-clients) over a pipe
rather than paying a sudo a second. Everything is also a gpu.* sensor.

What cannot be measured is left out, not shown as zero: an Arc A380 on i915 6.12
has no temperature sensor, an integrated GPU wears the package temperature
marked *, and Asahi has neither a busy counter nor fdinfo yet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

SENSORS.md

@@ -20,6 +20,7 @@ net.internet.download|upload|latency|jitter|tested|ip|edge
 disk.<dev>.read|write|util|size|temp
 fs.<mount>.size|used|used_pct
 ups.battery|runtime|load|load_pct|in_v|out_v|on_battery|state|source
+gpu.<name>.usage|render|video|video-enhance|copy|compute|temp|power|fan|freq|mem_used|mem_total|clients
 ```
 
 - Segments are lower case `[a-z0-9_-]`: `Package id 0` → `package_id_0`, `/mnt/tank` →
@@ -33,6 +34,14 @@ ups.battery|runtime|load|load_pct|in_v|out_v|on_battery|state|source
   has run on that machine, and each one's `t` is when it ran — so a widget or an alert on
   them is reading a measurement from that moment, not a live value. Nothing takes a speed
   test on its own; something has to press the button.
+- `gpu.<name>.*` is each graphics device, named as lspci names it (`gpu.intel_arc_a380`;
+  labels carry `driver` and the PCI `bus`). `usage` is how busy the device was — from the
+  GT's idle residency on Intel, `gpu_busy_percent` on amdgpu — and is absent on a driver
+  with no such counter. The per-engine percentages (`render`, `video`…) are the sum over the
+  processes holding the device open, read from DRM fdinfo, and exist only while something
+  has it open. `temp` is the card's own sensor, or on an integrated GPU the CPU package it
+  shares a die with (label `source` says which); an Arc on i915 has none and reports none.
+  `freq` is the actual clock, 0 when power-gated. `clients` counts the processes using it.
 
 ## A sensor