[Lethe10137]

• Bump tokio to 1.5x and refresh Cargo.lock; bump netem-trace to 0.4.5.
• Veth ingress: a larger send queue (try_reserve, drop on full instead of erroring), and skip the demux task when there is only one sender.
• Core router: use tokio::task::consume_budget() instead of yield_now().

Stack (merge in order, rebase-merge; review the diff against the base branch). This is 3/5:

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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