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/src/lib.rs

@@ -6,6 +6,7 @@ pub mod paths;
 pub mod process_watch;
 pub mod recommendation;
 pub mod power;
+pub mod tuning;
 pub mod types;
 
 pub use error::{MagnotiaError, Result};