* fix(tcp): stop the read-deadline re-arm from swallowing a cancelled ctx
the cancel watchdog trips conn.SetDeadline(now) once. if that trip
lands while the probe write is still in flight, readTCP's later
SetReadDeadline(now+timeout) call silently overwrote it, so the read
blocked for the full timeout instead of returning promptly. thread
ctx into readTCP and check it before arming the deadline and on every
read iteration so a cancellation already in flight aborts immediately.
* test(tcp): make the cancel-during-write test exercise the actual race
the old test cancelled ctx before calling ExecuteTCPModule, so the
pre-write ctx.Err() guard returned immediately and the test never
reached readTCP at all; reverting the fix and rerunning it still
passed in ~0ms. cancel from a goroutine mid-write instead, so the
watchdog trips the deadline while readTCP is the next call on the
path, and assert the call returns promptly rather than blocking for
the full timeout. also add a case proving a legitimately slow but
uncancelled connection still completes and matches normally.
