chore(core): add tuning_log_demo example for live heuristic validation

Run with:
  cargo run -p magnotia-core --example tuning_log_demo

Initialises a tracing-subscriber fmt layer so the inference_thread_count
INFO event is rendered to stderr. Exercises all 8 (workload,
gpu_offloaded) combos under three power scenarios: AC override,
battery override, and the real sysfs probe.

Used to validate the heuristic on 2026-05-09 against the spec's
6c12t truth table — all 6 rows match.

Adds tracing-subscriber as a [dev-dependencies] for the example.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/core/Cargo.toml

@@ -16,3 +16,4 @@ libloading = "0.8"
 
 [dev-dependencies]
 tempfile = "3"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }