Processes: CPU power beside the clock, from the packages' energy counters

New sensor cpu.power: watts from the RAPL energy counters (Intel, and AMD
Zen through the same driver), summed over every package and averaged into
history. Parts of a package, psys and the MMIO duplicate are left out.

The counters are root-only, so a daemon that isn't root reads them through
`hamlab cpu-energy`, a root helper started once with sudo or doas and asked
over a pipe each sample, the same shape as the GPU client walk's helper.
It reads only the counters it finds itself. No counters or no root means
no reading, and the CPU box shows nothing extra.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

SENSORS.md

@@ -12,7 +12,7 @@ added to the registry in `api.New`.
 ## IDs
 
 ```
-cpu.usage  cpu.core.<n>.usage  cpu.temp  load.1|5|15  uptime
+cpu.usage  cpu.core.<n>.usage  cpu.temp  cpu.power  load.1|5|15  uptime
 mem.total|used|avail|free|cached|buffers|used_pct     swap.total|used|avail|used_pct
 temp.<chip>.<label>            fan.<chip>.<label>
 net.<iface>.rx|tx|rx_total|tx_total|up
@@ -35,6 +35,11 @@ zfs.arc.size|target|min|max|mru|mfu|data|metadata|hit_pct|reads|misses|lifetime_
   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.
+- `cpu.power` is the CPU packages' own power in watts, from the kernel's RAPL energy
+  counters (Intel, and AMD Zen), summed over every package: cores, built-in graphics and
+  memory controller, not the whole machine. The counters are root-only, so a daemon that is
+  not root reads them through a helper started once with sudo or doas. It is absent on a
+  machine without the counters (ARM, the Mac mini) or without a way to root.
 - `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