[Lethe10137]

• rattan-env/src/netns.rs: set up the netns persist directory only once per process, serialized across processes with a file lock (/var/run/rattan-netns.lock).
• scripts/clean_stdenv.sh: unstack existing /var/run/netns mounts.

Stack (merge in order, rebase-merge; review the diff against the base branch). This is 4/5:

1. feat(loss): byte trigger for loss cells #217 feat(loss): byte trigger for loss cells
2. feat: first-payload start, ms granularity timestamp alignment, and drift log #218 feat: first-payload start, mahimahi-like delay, and drift log
3. perf(veth): update tokio and reduce ingress overhead #219 perf(veth): update tokio and reduce veth ingress overhead
4. 👉 fix(env): set up netns persist dir once per process #220 fix(env): set up the netns persist directory once per process
5. refactor(env): make Env generic and move I/O drivers into rattan-env #221 refactor(env): make Env generic and move I/O drivers into rattan-env

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