fix(output): route tracing to stderr so --output json stdout stays clean

common::output documents the contract: JSON events on stdout, banner + tracing
on stderr, so a parser reading stdout sees only events. But init_tracing relied
on tracing_subscriber::fmt()'s default writer, which is stdout — so every log
line was interleaved into the JSON event stream the --gui front-end parses.

The GUI tolerated it (non-JSON lines are skipped), but two real consequences:
a tracing write could corrupt a JSON event line intermittently, and all
diagnostics landed on stdout where the GUI discards them — leaving its
stderr-tail ring empty, so a failed host/viewer child surfaced no clue in the
window. Pin the fmt writer to stderr. Verified: every stdout line now parses as
JSON; iroh/tracing output appears on stderr.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main.rs

@@ -75,5 +75,14 @@ async fn main() -> Result<()> {
 fn init_tracing(verbose: bool) {
     let default = if verbose { "pixelpass=trace,iroh=info" } else { "pixelpass=info,iroh=warn" };
     let filter = EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new(default));
-    tracing_subscriber::fmt().with_env_filter(filter).with_target(false).init();
+    // Tracing MUST write to stderr. `tracing_subscriber::fmt()` defaults its
+    // writer to stdout, but with `--output json` stdout carries the JSON event
+    // stream the `--gui` front-end parses (see `common::output`) — logging there
+    // interleaves log lines into that stream (corrupting events and starving the
+    // GUI's stderr-tail diagnostics). Pin it to stderr to honor that contract.
+    tracing_subscriber::fmt()
+        .with_writer(std::io::stderr)
+        .with_env_filter(filter)
+        .with_target(false)
+        .init();
 }