chore: add startup benchmark script

[sacchen]

Summary

• Adds scripts/bench.sh to measure warm startup latency (p50/p95 over 12 runs)
• Covers four invocations: :version, 2+2, d(x^2,x), solve(x^2-4,x)

Current baseline (this machine)

:version                      p50=  223ms  p95=  246ms
2+2                           p50=  225ms  p95=  231ms
d(x^2, x)                     p50=  292ms  p95=  323ms
solve(x^2-4, x)               p50=  243ms  p95=  285ms

Findings from profiling (see issue #21 comment)

The dominant cost is sympy.polys (~118ms of SymPy's ~245ms total), loaded because solve needs polynomial algorithms. The urllib.request claim in the issue is not meaningful — its import is <0.1ms and the network call is already guarded by isatty().

Realistic floor with current feature set: ~200ms. The <150ms target in #21 is not achievable without removing solve from the namespace.

Closes #21 investigation phase.