feat(output): --output json machine-readable event stream

Adds common/output.rs: a process-global JSON-lines emitter for
non-interactive front-ends. With --output json, host and viewer emit one
JSON object per line on stdout (ticket, host_info, viewer_count, capture
start/stop, viewer_refused, connected), flushed per line; the human banner
and tracing logs stay on stderr so the two never interleave. No-op when the
flag is absent, so call sites emit unconditionally.

This is the shell-out counterpart to an in-process event channel: the
upcoming --gui front-end re-execs this binary as `pixelpass --host
--output json` and parses these lines to drive its window. serde_json was
already in the tree from the bandwidth pre-flight.

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

src/main.rs

@@ -16,6 +16,10 @@ async fn main() -> Result<()> {
     let cli = Cli::parse();
     init_tracing(cli.verbose);
 
+    if matches!(cli.output, Some(cli::OutputFormat::Json)) {
+        common::output::set_json(true);
+    }
+
     // libpipewire requires global init before any pw_* call. Idempotent;
     // safe to call even when the per-app audio thread never spawns.
     pipewire::init();