#2360 telemetry: the Mac's Monitor cards - GPU load and VRAM from IOKit, th…

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On a Mac the Monitor tab's GPU cards were empty: telemetry had NVML (NVIDIA) and the amdgpu sysfs files (#301) and nothing else, so the Metal backend read nothing.  The Apple GPU's load and memory are in the IOKit registry's IOAccelerator nodes (ioreg -l -w0 -r -c IOAccelerator, 22 ms), and the device's memory budget is the same Metal property the engine reads (metal/src/mtl_rt.mm's recommended_working_set, the engine's "GPU working set" log line
- 26,800,603,136 bytes here, which is 25.0 GiB).

_Metal (a new reader beside _Nvml and _Amd): the index'th accelerator node's PerformanceStatistics give Device Utilization % (older chips: the renderer or the tiler) and the in-use system memory; the total is the working set, read through the Metal framework and objc_msgSend (no PyObjC).  The temperature and power sensors are not readable without root (powermetrics), and an Apple Silicon GPU sits on the die, so there is no PCIe link: the cards show a dash and static gpu_note says why, the same way #1380's note does for a Windows AMD card.  gpu_reader takes `amd` first, so the AMD trees still read on a Mac for the tests, and a missing selector is asked for before it is sent (an unanswered one raises an Objective-C exception no ctypes call can catch).

Service.free_vram_mib now goes through gpu_reader instead of _Nvml directly, so the Mac gets that reading too (it used to return None there).

Measured on an M1 Max (macOS 26.6.2): through the server, GET /metrics during a request - gpu_name "Apple M1 Max", gpu_vram_total_mib 25559, gpu_util 74-77 % while decoding (9 % once idle), gpu_mem_used 10.5 GiB in use of 24.96 GiB, cpu 12.6 %; the plain-text /prometheus-format metrics carry the same series.  serve/test_server.py: MacTelemetry (fixture registry: two accelerators, index selection, renderer/tiler fallback, a raising runner, the note and the samples) plus AmdTelemetry, TelemetryNoDisks, Monitor and the full serve/test_server.py (318 tests) pass on the same box.

What it leaves alone: the NVIDIA and AMD paths (a test-only gpu_reader order change on macOS), and the web app - the cards read the series it already had.

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