Broker: avoid EPOLLOUT add/remove churn per write (+60-90% throughput)

[dgprivate]

Broker: avoid EPOLLOUT add/remove churn on every write

Summary

packet__write() (broker build) registers EPOLLOUT via mux__add_out() at the
top of the function and removes it again via mux__remove_out() once the out
queue drains. In the common case — the socket accepts the write synchronously —
this costs two epoll_ctl() syscalls per write cycle that the event loop
never needed.

This changes it to the standard level-triggered pattern: only register EPOLLOUT
when a write actually blocks (EAGAIN with data still pending). The outgoing
bridge connect_pending case still arms EPOLLOUT explicitly, and packet__queue
already attempts an eager write for every queued packet, so nothing can be left
waiting for an EPOLLOUT that was never armed.

Deterministic effect (strace -c -e epoll_ctl, fixed 200k-message run)

build	epoll_ctl calls	per message
before	334,728	~1.67
after	2	~0

perf before: epoll_ctl = ~9.6% of broker CPU; after: absent from the profile.

Throughput (interleaved A/B, broker pinned to one core, plain TCP, 64B QoS0)

scenario	before	after	delta
1 pub → 1 sub	304k/s	547k/s	+80%
8 pub → 2 sub	129k/s	249k/s	+93%
4 pub → 4 sub	50k/s	86k/s	+72%
2 pub → 8 sub	24k/s	39k/s	+64%
8 pub → 8 sub	36k/s	58k/s	+60%

(Trials alternated between two fixed binaries built from the parent commit and
this commit, to control for host drift.)

Correctness

• Backpressure — 100 KB QoS1 messages to a deliberately slow reader, all
  delivered intact and in order over plain TCP, TLS (SSL_write WANT_WRITE),
  and builtin WebSockets.
• Fan-out 1→40 subscribers: every subscriber received all messages, no
  corruption.
• poll backend (WITH_EPOLL=no) exercised as well as epoll.
• No busy-loop: broker CPU stays low while backpressured.
• Broker test suite: unchanged pass/fail versus the parent commit (including the
  bridge tests that cover the connect_pending path).

Second commit (optional)

The second commit adds a small throughput benchmark under misc/bench/ used to
produce the numbers above. Happy to drop it if you'd rather keep it out of tree.