psmux 13386a0ba1 Add performance benchmark suite (tests/bench)
Statistically rigorous, percentile-based benchmarks executed serially in a
clean environment (server killed and process table verified between runs) so
concurrent psmux launches never contaminate timing.

- bench_startup.ps1: cold/warm session startup (15 cold + 25 warm iters)
- bench_input_latency.ps1: keystroke-to-echo latency (idle/under-load/scrollback)
- bench_throughput.ps1: output rendering throughput (bulk/ANSI/rapid-burst)
- bench_scaling.ps1: window/pane/session scaling + memory profile + leak check
- bench_ipc_compare.ps1: CLI and raw-TCP IPC latency + launch comparison
- bench_wt_compare.ps1: controlled launch-to-visible-window vs Windows Terminal
  and pwsh (opens and closes only its own windows; never kills WindowsTerminal)
- BENCHMARK_RESULTS.md: methodology and run1 results

run1 (Ryzen AI Max+ 395): cold launch 71ms server / 388ms prompt, warm 1ms,
one-shot IPC sub-millisecond, attached-client render tick ~15.5ms, output
~10k lines/s, window create 24ms, ~0.28MB/window, no leak.
2026-06-13 07:16:53 +05:30
..