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>

PLAN.md

@@ -299,6 +299,21 @@ something long in one, switch away and back, refresh — it's all still there.
   id and its route are gone. A rail order saved before this keeps its shape — `railorder.ts`
   maps the retired `top` to `procs` and drops the duplicate, so the merged page sits where
   Top sat rather than falling to the bottom.
+- **GPUs on the Processes tab** — **shipped 2026-10-02** (`internal/metrics/gpu.go`, the
+  `GPUBox` in `Top.tsx`, `gpu.*` sensors). Asked for as "GPU activity and temps, like btop
+  has it, always on". Each DRM device with a real GPU driver gets a box under the cpu box:
+  busy curve, busy meter, a meter per engine (render/video/copy/enhance), temperature, clock,
+  power and fan where the card has them, and the processes that have it open with their
+  share. The process table gains a `Gpu%` column and sort. **How it is measured, and the
+  honest gaps:** device busy is Intel's RC6 idle residency inverted (the counter every GT
+  exposes without root; `intel_gpu_top` shows the same number) or amdgpu's own percent;
+  per-process and per-engine shares are DRM fdinfo deltas, the way nvtop reads them. Other
+  users' fdinfo needs root, so a daemon running as `core` keeps **one** root child
+  (`hamlab drm-clients`, over a pipe) rather than a sudo a second. The Arc A380 on i915 6.12
+  exposes power, energy and fan but **no temperature**, so its box shows none; an integrated
+  GPU shows the CPU package temperature marked `*`. Asahi (the Mac mini) has neither a busy
+  counter nor fdinfo in 6.19, so it lists the device and says so. No NVIDIA path: nothing here
+  has one to test against.
 - **Dashboard retired** — **2026-09-23**, at Mitchell's instruction, the same reasoning that
   merged Top into Processes: the app repeated itself. The customisable module list is gone
   (`Dashboard.tsx` deleted), and with it `/api/prefs/dashboard`, `applyPrefs`, and the