fix(torrent): don't lose msg writer wakeups during buffer fill

[aslafy-z]

Fixes #1070.

Change

Two changes in peer-conn-msg-writer.go's run loop:

• Obtain writeCond.Signaled() before fillWriteBuffer(): a Broadcast landing mid-fill now closes the pre-installed channel instead of being dropped, so the park returns immediately.
• Arm keepAliveTimer before each park with the time remaining until the keepalive deadline, falling back to a full interval once it has passed. The timer was previously only Reset after a successful write, so a bare fire on an idle connection left it dead, removing the periodic backstop; per-park arming also keeps the deadline accurate instead of restarting the interval on every wakeup.

Note for review: Signaled() is now called without holding cn.mu (it previously ran under it in the park branch). That's safe: BroadcastCond is internally synchronized — lock-free in chansync v0.8.0 (atomic.Pointer + CAS) — so concurrent Signaled/Broadcast need no external locking. The type's doc line about callers bringing their own synchronization concerns coordinating the guarded state (it explains why there is no Signal), not these calls.

Tests

Each change is pinned by its own deterministic test; both fail on master with distinct diagnostics and pass under -race with the fix:

• TestMsgWriterTickleDuringFillNotLost: holds the writer inside fillWriteBuffer and issues the Broadcast strictly within that window. Without the fix there is no subscriber yet and the writer parks forever.
• TestMsgWriterKeepAliveTimerRearmed: lets the timer fire once on a not-yet-useful connection, then makes it useful; no tickle is involved, so the keepalive can only come from a re-armed timer. Without the fix the timer stays dead.

External validation

streamline's bittorrent integration suite (28 specs, including a 30-download-cycle spec that reliably wedged under CPU contention on v1.61) passes against this fix with their engine-side 5s-keepalive mitigation (DataHearth/streamline@8521da1) reverted: DataHearth/streamline#4.