feat(connection): add explicit PauseRead/ResumeRead/IsReadPaused APIs

[NickPak]

Expose application-driven read backpressure on the Connection interface:

• PauseRead() stop monitoring readable (EPOLLIN) events for this conn
• ResumeRead() resume monitoring readable events
• IsReadPaused() report whether readable monitoring is currently paused

These are minimal mechanisms that let the application decide when to apply TCP backpressure based on its own signal (downstream/outbound queue depth, work-queue high/low watermarks, credits, ...), instead of relying on netpoll's inbound LinkBuffer size, which is not a reliable proxy for application overload (e.g. proxies that drain the inbound buffer quickly into their own queue).

Implementation notes:

• Read and write interest are now two independent dimensions. The poll mask is recomputed from (readPaused, writeWatching) on every transition; adds PollR2N/PollN2R/PollN2W/PollW2N alongside the existing PollRW2W/PollW2RW so read can be toggled without disturbing a pending write and vice versa.
• When read is paused and there is no pending write, the connection drops to "no interest" rather than write-only, avoiding a level-triggered EPOLLOUT busy-spin on receive-mostly connections.
• Transitions are serialized with a per-connection mutex: the two dimensions are mutated by different goroutines (application vs poller) and epoll_ctl(MOD) sets an absolute mask, so atomics alone cannot order "state change + syscall".
• PauseRead/ResumeRead are guarded by IsActive() to avoid calling Control() on a freed operator after close.
• epoll (Linux) and kqueue (BSD) control paths updated.
• Added TestConnectionPauseResumeRead.

No behavior change unless the new APIs are called.

[CLAassistant]

All committers have signed the CLA.