Phase 10: Monitoring and automated benchmarks

10.1: Add Criterion dev-dependency and benchmarks for:
  - matching_rules (100 rules)
  - ComputedStyle::compute (empty, 5 props)
  - resolve_string (no vars, 1 var, 3 vars)

10.3: Add --perf CLI flag and src/perf.rs module:
  - PerfScope drop-guard timed scopes (vdom, style, render)
  - Instrumented: VDOM eval (app.rs update), render (app.rs view),
    style matching (style.rs compute_cached)
  - Per-frame report printed to stderr at end of view()

10.2+10.4: Threshold check in perf report:
  - Prints warning if total > 16ms (60 FPS frame budget)

Refactor: Move Message enum to lib.rs so benchmarks can import
the public API. Added lib.rs as library root.

benches/bench.rs

@@ -0,0 +1,99 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+
+fn bench_matching_rules(c: &mut Criterion) {
+    let mut group = c.benchmark_group("matching_rules");
+    group.sample_size(10);
+
+    group.bench_function("100_rules", |b| {
+        b.iter(|| {
+            let mut stylesheet = glint_runtime::interpreter::style::StyleSheet::new();
+            for i in 0..100 {
+                stylesheet.add_rule(
+                    format!(".class-{}", i),
+                    std::collections::HashMap::from([("color".into(), "red".into())]),
+                );
+            }
+            stylesheet.build_index();
+            let _ = stylesheet.matching_rules(
+                "Button",
+                None,
+                &["class-1"],
+                &[],
+                &Default::default(),
+                &[],
+                &[],
+                &std::collections::HashMap::new(),
+            );
+        })
+    });
+
+    group.finish();
+}
+
+fn bench_style_compute(c: &mut Criterion) {
+    let mut group = c.benchmark_group("computed_style");
+    group.sample_size(10);
+
+    group.bench_function("compute_empty", |b| {
+        b.iter(|| {
+            let _ = glint_runtime::interpreter::style::ComputedStyle::compute(
+                &[],
+                &[],
+            );
+        })
+    });
+
+    group.bench_function("compute_5_props", |b| {
+        let props: Vec<(std::borrow::Cow<'_, str>, std::borrow::Cow<'_, str>)> = vec![
+            ("color".into(), "red".into()),
+            ("padding".into(), "10px".into()),
+            ("font-size".into(), "16px".into()),
+            ("background".into(), "#fff".into()),
+            ("width".into(), "100px".into()),
+        ];
+        b.iter(|| {
+            let _ = glint_runtime::interpreter::style::ComputedStyle::compute(
+                &props,
+                &[],
+            );
+        })
+    });
+
+    group.finish();
+}
+
+fn bench_resolve_string(c: &mut Criterion) {
+    let mut group = c.benchmark_group("resolve_string");
+    group.sample_size(10);
+
+    let vars = std::collections::HashMap::from([
+        ("name".to_string(), glint_runtime::interpreter::Value::Str("world".into())),
+        ("count".to_string(), glint_runtime::interpreter::Value::Int(42)),
+    ]);
+
+    group.bench_function("no_vars", |b| {
+        b.iter(|| {
+            use glint_runtime::interpreter::Interpreter;
+            let _ = Interpreter::resolve_string("hello world", &vars);
+        })
+    });
+
+    group.bench_function("1_var", |b| {
+        b.iter(|| {
+            use glint_runtime::interpreter::Interpreter;
+            let _ = Interpreter::resolve_string("Hello $name!", &vars);
+        })
+    });
+
+    group.bench_function("3_vars", |b| {
+        b.iter(|| {
+            use glint_runtime::interpreter::Interpreter;
+            let _ = Interpreter::resolve_string("$name has count $count and is $name", &vars);
+        })
+    });
+
+    group.finish();
+}
+
+criterion_group!(benches, bench_matching_rules, bench_style_compute, bench_resolve_string);
+criterion_main!(benches);