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"] }

crates/core/examples/tuning_log_demo.rs

@@ -0,0 +1,50 @@
+//! Demonstrator: show what the inference_thread_count tracing event
+//! emits in production, across all eight (workload, on_battery,
+//! gpu_offloaded) tuples.
+//!
+//! Run with:
+//!   cargo run -p magnotia-core --example tuning_log_demo
+//!
+//! Output is to stderr (tracing's default). Each unique tuple emits
+//! exactly one INFO line; subsequent calls with the same tuple are
+//! silenced by the per-process log-once cache.
+
+use magnotia_core::tuning::{inference_thread_count, Workload};
+
+fn main() {
+    tracing_subscriber::fmt()
+        .with_env_filter("magnotia_core=info")
+        .with_target(true)
+        .with_writer(std::io::stderr)
+        .init();
+
+    let cores = num_cpus::get_physical();
+    let logical = num_cpus::get();
+    eprintln!("Host: {cores} physical / {logical} logical cores\n");
+
+    for (label, override_value) in [
+        ("AC override", Some("ac")),
+        ("Battery override", Some("battery")),
+        ("No override (real sysfs probe)", None),
+    ] {
+        match override_value {
+            Some(v) => std::env::set_var("MAGNOTIA_POWER_STATE_OVERRIDE", v),
+            None => std::env::remove_var("MAGNOTIA_POWER_STATE_OVERRIDE"),
+        }
+        // Cache invalidation so the live probe re-runs each section.
+        // Override paths bypass the cache anyway; this is for the
+        // no-override block that actually hits sysfs.
+        eprintln!("--- {label} ---");
+        for &(workload, gpu) in &[
+            (Workload::Llm, false),
+            (Workload::Llm, true),
+            (Workload::Whisper, false),
+            (Workload::Whisper, true),
+        ] {
+            let n = inference_thread_count(workload, gpu);
+            let w = format!("{workload:?}");
+            eprintln!("  {w:>8} gpu_offloaded={gpu:>5} -> {n} threads");
+        }
+        eprintln!();
+    }
+}