perf(tfork): replace pidfile polling with runtime readiness events- #14

[yiying-zhang]

Summary

• add a CRIU tfork-source-detached notification;
• forward readiness through crun to Podman using a file descriptor;
• replace the 200-ms pidfile polling loop with event-driven readiness;
• retain timeout, lost-event, and failure fallback behavior;
• gate the new path with PODMAN_TFORK_EVENT_READINESS=1.

Performance

With source syncfs, socket-scan overlap, and an independent cgroup-freeze
monitor enabled in both arms:

Mode	Wall median	Mean	p90
polling	0.840 s	0.769 s	0.860 s
events	0.655 s	0.654 s	0.670 s

The event path removes the quantized 200-ms tail. All 40 monitored and
unmonitored forks passed.

Source-pause finding

Measured source frozen time was effectively unchanged: 107.20 ms with polling
and 109.11 ms with events. CRIU already unfroze the inner source cgroup; the
later outer Podman thaw was redundant. This change improves child-ready wall
latency and makes readiness explicit, but it does not claim a source-pause
reduction on this stack.