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/cli.rs

@@ -68,6 +68,12 @@ pub struct Cli {
     pub port: u16,
 
     // ── global ────────────────────────────────────────────────────────
+    /// Emit machine-readable events on stdout (one JSON object per line)
+    /// alongside the human banner on stderr. For scripts and the --gui
+    /// front-end. Currently only `json` is supported.
+    #[arg(long, value_enum, value_name = "FORMAT")]
+    pub output: Option<OutputFormat>,
+
     /// Trace-level logging.
     #[arg(long, short)]
     pub verbose: bool,
@@ -89,6 +95,12 @@ pub enum DisplayServerArg {
     X11,
 }
 
+#[derive(ValueEnum, Clone, Copy, Debug, PartialEq, Eq)]
+pub enum OutputFormat {
+    /// One JSON object per line on stdout.
+    Json,
+}
+
 /// Quality preset. Each fixed preset bundles a (max-height, bitrate, fps)
 /// tuple — resolution is a quality-per-bitrate knob, so the three only make
 /// sense together. `Auto` has no fixed tuple; it picks one of the others from