Exclude non-hybrid CPUs from intel-lpmd via cpu_hybrid check

[hotline1337]

Summary

• Adds a generic cpu_hybrid profile key: cpu_hybrid = true only matches systems whose CPU is hybrid (has E-cores).
• Sets cpu_hybrid = true on the intel-lpmd profile, so it no longer matches non-hybrid CPUs that share the same cpu_family/cpu_models.
• Does not carry over the blacklisted_device_ids approach from Add blacklisted IDs and exclude non-hybrid CPUs from intel-lpmd #286.

Context

This is a rework of #286 based on feedback and discussion on Discord.

The previous approach (maintaining a device blacklist) was rejected as fragile and unmaintainable. Instead, as suggested by maintainers, this PR implements a generic detection mechanism using the Atom PMU sysfs interface (/sys/devices/cpu_atom).

How the check works

On hybrid CPUs the kernel registers the Atom (E-core) perf PMU under /sys/devices/cpu_atom, next to the cpu_core (P-core) PMU, while non-hybrid CPUs only expose the unified cpu PMU.

The directory alone is not a reliable signal, though: non-hybrid dies of hybrid SoCs (e.g. Alder Lake-S without E-cores, such as the i5-12400F) still expose /sys/devices/cpu_atom, but their cpus file contains no CPU at all (just a line feed). hwd_misc::is_hybrid_cpu() therefore reads /sys/devices/cpu_atom/cpus, and requires the file to actually contain a CPU number; a missing or unreadable file counts as non-hybrid.

Changes

1. src/hwd_misc.rs: added is_hybrid_cpu().
2. src/profile.rs: cpu_hybrid is read in parse_profile() and emitted in profile_into_toml().
3. src/data.rs: profiles with cpu_hybrid set are only matched when the detected hybridness agrees.
4. profiles/pci/power_management/profiles.toml: intel-lpmd sets cpu_hybrid = true.
5. tests/profiles/cpu-profile-test.toml + unit tests for the detection (with E-cores / without E-cores / missing PMU).

Verification

• cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings and cargo test all pass.
• Verified on an i5-12400F (6P+0E, non-hybrid): /sys/devices/cpu_atom/cpus contains only 0x0A and is_hybrid_cpu() returns false, so intel-lpmd is no longer offered there.