feat(tracing): add performance profiling via tracing crate

Wire up the existing --profile flag with tracing-subscriber to emit
span timings on stderr. Supports MOZART_LOG env var override and
verbose-level-aware filtering. No subscriber is installed when
--profile is off, keeping tracing macros zero-cost.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/crates/mozart/src/main.rs b/crates/mozart/src/main.rs
index 59ad392..ebe84e5 100644
--- a/crates/mozart/src/main.rs
+++ b/crates/mozart/src/main.rs
@@ -1,9 +1,45 @@
 use clap::Parser;
 use mozart::commands;
 use mozart_core::exit_code;
+use tracing_subscriber::{fmt, prelude::*, EnvFilter};
+
+fn init_tracing(profile: bool, verbose: u8, quiet: bool) {
+    // MOZART_LOG environment variable takes highest priority.
+    if let Ok(env_filter) = EnvFilter::try_from_env("MOZART_LOG") {
+        tracing_subscriber::registry()
+            .with(fmt::layer().with_writer(std::io::stderr))
+            .with(env_filter)
+            .init();
+        return;
+    }
+
+    if profile {
+        let filter = match verbose {
+            0 => "mozart=info",
+            1 | 2 => "mozart=debug",
+            _ => "mozart=trace",
+        };
+        tracing_subscriber::registry()
+            .with(
+                fmt::layer()
+                    .with_writer(std::io::stderr)
+                    .with_timer(fmt::time::uptime())
+                    .with_span_events(fmt::format::FmtSpan::CLOSE),
+            )
+            .with(EnvFilter::new(filter))
+            .init();
+    } else if verbose >= 3 && !quiet {
+        tracing_subscriber::registry()
+            .with(fmt::layer().with_writer(std::io::stderr).with_target(false))
+            .with(EnvFilter::new("mozart=debug"))
+            .init();
+    }
+    // Otherwise: no subscriber installed → tracing macros are effectively zero-cost no-ops.
+}
 
 fn main() {
     let cli = commands::Cli::parse();
+    init_tracing(cli.profile, cli.verbose, cli.quiet);
     match commands::execute(&cli) {
         Ok(()) => {}
         Err(e) => {