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"