Concrete perf metrics: live counters in --profile + benchmark suite

Live metrics (--profile):
- New metricsTracker instruments OnPTYOut, viewport renderer,
  stdout writes, libghostty-vt Write/Title CGO calls, sidebar /
  tabbar / status draws (with cache-hit accounting), snapshot
  replays, and the chrome ticker (so we can see ticker fires that
  did nothing).
- Writes metrics.jsonl (one snapshot per second) and metrics.json
  + summary.txt on exit, alongside the existing pprof files.
- All record* methods are nil-safe so disabled paths pay only a
  cheap nil check; counters are atomic so the per-PTY-chunk hot
  path stays lock-free.

Benchmark suite (go test -bench=.):
- Three workload fixtures — plain ASCII, SGR-styled lines, and a
  ratatui-style cursor-shuffling burst — plus a containsOSC
  microbenchmark. Reports ns/op, MB/s, allocs/op, B/op.
- Initial baseline numbers added to TODO under the perf-audit
  section, alongside two new findings (renderer allocs ~1 per 4
  bytes on styled chunks; styled throughput tops out near
  90 MB/s) those benchmarks surfaced.

internal/app/sidebar.go

@@ -38,6 +38,10 @@ func formatShortDuration(d time.Duration) string {
 // computed main viewport, so the sidebar region is outside the child's
 // cursor range. We can redraw freely without fighting the child for cells.
 func (st *uiState) drawSidebar() {
+	var entry time.Time
+	if st.metrics != nil {
+		entry = time.Now()
+	}
 	st.mu.Lock()
 	palOpen := st.palette != nil
 	focus := st.focusedID
@@ -231,10 +235,16 @@ func (st *uiState) drawSidebar() {
 	st.chromeCacheMu.Lock()
 	if frame == st.sidebarCache {
 		st.chromeCacheMu.Unlock()
+		if st.metrics != nil {
+			st.metrics.recordSidebar(time.Since(entry), true)
+		}
 		return
 	}
 	st.sidebarCache = frame
 	st.chromeCacheMu.Unlock()
+	if st.metrics != nil {
+		defer func() { st.metrics.recordSidebar(time.Since(entry), false) }()
+	}
 
 	st.outMu.Lock()
 	// Save cursor; emit the sidebar; restore.