hal/temperature: correct sysfs-hwmon temperature fallback for Arc Pro…

[thevisad]

Description

xpu-smi reports GPU Core and Memory Temperature as N/A on the Intel Arc Pro B70 (Battlemage, xe
driver): Level Zero sysman enumerates no GPU/Memory temperature sensor on this device, even though the
readings are published on the PCI device's hwmon node (labels pkg / vram). This PR adds a correct,
validated sysfs-hwmon fallback so temperature is reported on such devices, while leaving behaviour
byte-identical where Level Zero already provides a sensor.

What it does

• getCoreTemp / getMemoryTemp fall back to sysfs only when Level Zero reports the sensor type
  unsupported (no matching sensor was enumerated); any other Level Zero result is propagated, so
  device-loss / permission / state errors are never masked. The routing decision is a pure, unit-tested
  helper (decideTempSource).
• getTempPerTile uses the fallback only when no matching-type sensor was enumerated, not merely when the
  tile map is empty (perTileShouldFallback).
• resolveSysfsHwmon scans all hwmon* subdirs and caches the exact tempN_input path per label;
  strict temp<N>_label filename matching; every stream open is verified.
• One shared, bounds-checked read path (parse → milli→Celsius → physical-validity bounds). The lower
  bound is absolute zero, not a device-specific 0 C floor, so a legitimate subzero reading on another
  matching system is not wrongly rejected.
• The pure logic lives in an internal xpum::hwmon utility (hwmon_temperature_utils.{h,cpp}) shared by
  temperature.cpp and the tests, without expanding the exported temperature class / DLL surface.

Testing

• doctest: 15 cases / 87 assertions (decision matrix SUCCESS→L0 / UNSUPPORTED→sysfs /
  DEVICE_LOST,PERMISSIONS,UNKNOWN→propagate; strict filename parse; value bounds; multi-hwmon
  discovery incl. labelled node not first; two roots don't cross-resolve). Full meson test: 9/9 suites.
• Hardware (Arc Pro B70, xe, kernel 7.0.0-28-generic, Level Zero 1.27.0): xpu-smi stats reports
  Core 51 / Mem 52, matching raw sysfs pkg=51000 / vram=52000; VR stays N/A.

Scope / limitations

• Validated only on the tested single-tile Arc Pro B70 on xe. The fallback is condition-based, not
  PCI-ID-gated: it activates when Level Zero exposes no matching sensor and the hwmon node provides the
  expected label. Not generalized to multiple tiles/subdevices; VR temperature is not in sysfs and stays
  N/A.
• No unguarded POSIX-only header remains (directory scans and test fixtures use std::filesystem).
  Windows was not build-tested. The doctest test targets require a doctest package in CI.

Fixes:
Relates-To:

Type of change

• Bug fix
• New feature
• API / ABI change
• Refactor / cleanup
• Documentation
• Tests
• Dependency update
• CI / build
• Other

Affected components

• hal — Hardware Abstraction Layer
• ial — Interface / Application Layer
• oal — OS Abstraction Layer
• xpumd — XPU Manager Daemon
• cli / smi
• ci / build system

Checklist

• Commit messages follow the project guidelines
• Each commit includes Signed-off-by (DCO)
• New public APIs or CLI options are documented — n/a (no new public API/CLI; internal helpers only)
• No unrelated changes included
• Squash "fixup" commits after reviews — n/a (single commit)

[savery42]

Related to #146