feat(core): tuning module with Workload + helper skeleton

Adds crates/core/src/tuning.rs with MIN/MAX_INFERENCE_THREADS consts,
Workload enum (Llm/Whisper), and inference_thread_count() helper matching
the existing constants::inference_thread_count clamp behaviour. Three unit
tests pass. tracing dep added to Cargo.toml (used by future tasks).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/core/Cargo.toml

@@ -11,6 +11,7 @@ thiserror = "2"
 sysinfo = "0.35"
 async-trait = "0.1"
 num_cpus = "1"
+tracing = "0.1"
 
 [dev-dependencies]
 tempfile = "3"