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.

cmd/patterm/main.go

@@ -53,7 +53,7 @@ func main() {
 		projectDir  = flag.String("project", "", "project directory (default $PWD)")
 		showVersion = flag.Bool("version", false, "print version and exit")
 		debugDir    = flag.String("debug", "", "write debug logs + per-child raw PTY output to DIR (auto-picks a dated subdir under $XDG_STATE_HOME/patterm/debug when DIR is omitted)")
-		profileDir  = flag.String("profile", "", "write CPU+heap+goroutine pprof files to DIR (auto-picks a dated subdir under $XDG_STATE_HOME/patterm/profile when DIR is omitted)")
+		profileDir  = flag.String("profile", "", "write pprof files (cpu/heap/goroutine) and live perf counters (metrics.jsonl per-second, metrics.json + summary.txt on exit) to DIR (auto-picks a dated subdir under $XDG_STATE_HOME/patterm/profile when DIR is omitted)")
 	)
 	// Allow bare `--debug` / `--profile` with no value — pflag treats
 	// them as boolean-shaped strings, picking a sensible default dir.
@@ -99,6 +99,7 @@ func main() {
 		ProjectDir: cwd,
 		ProjectKey: key,
 		DebugDir:   resolvedDebug,
+		ProfileDir: resolvedProfile,
 	}); err != nil {
 		die("%v", err)
 	}