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/tabbar.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"time"
 	"unicode/utf8"
 )
 
@@ -17,6 +18,10 @@ const tabBarRows = 2
 // to the leftmost tabs so the strip fills the screen edge-to-edge.
 // A trailing "+ new" hint sits in the rightmost reserved slot.
 func (st *uiState) drawTabBar() {
+	var entry time.Time
+	if st.metrics != nil {
+		entry = time.Now()
+	}
 	st.mu.Lock()
 	palOpen := st.palette != nil
 	focus := st.focusedID
@@ -188,10 +193,16 @@ func (st *uiState) drawTabBar() {
 	st.chromeCacheMu.Lock()
 	if frame == st.tabBarCache {
 		st.chromeCacheMu.Unlock()
+		if st.metrics != nil {
+			st.metrics.recordTabbar(time.Since(entry), true)
+		}
 		return
 	}
 	st.tabBarCache = frame
 	st.chromeCacheMu.Unlock()
+	if st.metrics != nil {
+		defer func() { st.metrics.recordTabbar(time.Since(entry), false) }()
+	}
 
 	st.outMu.Lock()
 	defer st.outMu.Unlock()