Harden UDP parsing and speed up reactor for 10k conns

[treeform]

Summary

• Harden untrusted UDP receive (length checks, caps, no wire asserts, close(), README honesty).
• Speed up hot paths: connection Table, receive window, send Deque/part pool, ACK bundling, larger per-tick drain.
• Add reactor bench with 10k default scale (Istrolid1 peaked ~3k; Istrolid2 headroom). Baseline in docs/bench-baseline.md.
• DefaultMaxConnections = 10_000.

Test plan

• nim r tests/tests.nim
• nim r tests/test_hardening.nim
• nim r -d:release -d:nettyBench tests/bench_reactor.nim (default 10k)
• Compare output to docs/bench-baseline.md