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>

Cargo.lock

@@ -2862,6 +2862,7 @@ dependencies = [
  "tempfile",
  "thiserror 2.0.18",
  "tracing",
+ "tracing-subscriber",
 ]
 
 [[package]]
@@ -2973,6 +2974,15 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "matchers"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d1525a2a28c7f4fa0fc98bb91ae755d1e2d1505079e05539e35bc876b5d65ae9"
+dependencies = [
+ "regex-automata",
+]
+
 [[package]]
 name = "matches"
 version = "0.1.10"
@@ -3238,6 +3248,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "nu-ansi-term"
+version = "0.50.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
+dependencies = [
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "num-complex"
 version = "0.4.6"
@@ -4964,6 +4983,15 @@ dependencies = [
  "digest",
 ]
 
+[[package]]
+name = "sharded-slab"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f40ca3c46823713e0d4209592e8d6e826aa57e928f09752619fc696c499637f6"
+dependencies = [
+ "lazy_static",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -6031,6 +6059,15 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "thread_local"
+version = "1.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f60246a4944f24f6e018aa17cdeffb7818b76356965d03b07d6a9886e8962185"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "tiff"
 version = "0.11.3"
@@ -6344,6 +6381,35 @@ dependencies = [
  "valuable",
 ]
 
+[[package]]
+name = "tracing-log"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ee855f1f400bd0e5c02d150ae5de3840039a3f54b025156404e34c23c03f47c3"
+dependencies = [
+ "log",
+ "once_cell",
+ "tracing-core",
+]
+
+[[package]]
+name = "tracing-subscriber"
+version = "0.3.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb7f578e5945fb242538965c2d0b04418d38ec25c79d160cd279bf0731c8d319"
+dependencies = [
+ "matchers",
+ "nu-ansi-term",
+ "once_cell",
+ "regex-automata",
+ "sharded-slab",
+ "smallvec",
+ "thread_local",
+ "tracing",
+ "tracing-core",
+ "tracing-log",
+]
+
 [[package]]
 name = "transcribe-rs"
 version = "0.3.11"

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!();
+    }
+}