Adds a OnceLock<Mutex<HashSet>> cache in tuning.rs. inference_thread_count
now emits a single tracing::info! line the first time each
(workload, on_battery, gpu_offloaded) tuple is seen in the process,
recording the resolved thread count and which clamps fired (battery, gpu).
Also derives Hash on Workload and tightens the GPU clamp to only record
the "gpu" clamp when it actually reduces chosen. Smoke test added.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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