perf: push socket events host→guest instead of guest wait=0 polling

Problem

Guest networking pays a ~2–4ms quantum per socket event (accept, data, close), because the embedded node polyfill (crates/execution/src/node_import_cache.rs:4750-4752) calls the net.poll / net.server_poll sync RPCs with waitMs = 0 and paces itself with guest-side setTimeout timers (scheduleSocketPoll). Latency is the guest timer cadence, not sidecar work.

Measured 2026-07-01 (3-layer differential bench, p50, guest VM vs host node — same program):

Cost scales with event count (one accept = one ~4ms tick; a full echo ≈ 5 events ≈ 21ms), not payload size. Sidecar-side wait improvements (#172, #173) are correct but cold on this path — the guest always passes wait==0, so the sidecar never waits.

Constraint — why not waitMs > 0

The polyfill cannot block: callSync parks the whole V8 isolate. And the sidecar services these RPCs synchronously on a single-thread tokio runtime (new_current_thread().block_on, stdio.rs), so a wait>0 sync RPC would park the entire sidecar event loop and serialize all VM traffic behind it. Event push is the only shape that doesn't block either loop.

Fix direction

Push socket readiness/connection events host→guest asynchronously over the stream-callback channel the bridge already uses for stdio, and have the polyfill resolve pending accepts/reads/writes off those events instead of re-arming timers. The sidecar-side readiness plumbing from #172 (wait_fd_readable_until) already gives the sidecar an efficient way to learn about readiness to forward.

Follow-up (smaller, same theme)

Sidecar-internal waits can also go push-based: e.g. the loopback peer-pairing wait (#173, currently deadline+backoff polling) could subscribe to a socket-table-insert notification, using the same Condvar/Notify + bounded-timeout-fallback pattern as #174.

Regression gates

Standing bench rows in agent-os scripts/benchmarks fuzz-perf lane: net/udp_echo_small, net/unix_echo_small, net/http_loopback_get, net/tcp_*, perf-finding/unix_accept_latency (agentos#1570/#1571).

Start with crates/execution/src/node_import_cache.rs:4750-4752 and trace the net.poll and net.server_poll calls, including scheduleSocketPoll and the stream-callback channel used for stdio. Then inspect the sidecar readiness path from #172. Done means socket events reach the guest without timer re-arming and the listed agent-os benchmark rows improve without breaking networking behavior.