Adds CDPSupervisor.evaluate_runtime() and wires it into _browser_eval as a
fast path when a supervisor is alive for the current task_id. Replaces the
~180ms agent-browser subprocess fork+exec+Node-startup hop with a ~1ms
Runtime.evaluate over the supervisor's already-connected WebSocket.
Falls through to the existing agent-browser CLI path when no supervisor is
running (e.g. backends without CDP, or before the first browser_navigate
attaches one), so behaviour is unchanged where it can't apply.
JS-side exceptions surface directly without falling through to the
subprocess (the subprocess would just re-raise the same error, slower);
supervisor-side failures (loop down, no session) fall through cleanly.
Benchmark — 30 iterations of `1 + 1` against headless Chrome:
supervisor WS mean= 0.96ms median= 0.91ms
agent-browser subprocess mean=179.35ms median=167.73ms
→ 187x speedup mean
Tests: 14 unit tests (mocked supervisor + response-shape coverage), 5
real-Chrome e2e tests in test_browser_supervisor.py (gated on Chrome
being installed). Browser test suite: 355 passed, 1 skipped.
