DF-0596 FIXED: xmitWin growth with clamp + OOB prevention
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Layout: &timeSent[16]=offset 200, &recvSeq=offset 200 -> OVERLAP (OOB target)

[threads=2] 10000 rounds: xmitWin>16 in 0 rounds, max_xw=16
[threads=4] 10000 rounds: xmitWin>16 in 0 rounds, max_xw=16
[threads=8] 10000 rounds: xmitWin>16 in 0 rounds, max_xw=16

=== FIXED RESULTS ===
Total over-16: 0, max_xw=16 (PPTP_XMIT_WIN=16)
*** FIX WORKS: xmitWin clamped to PPTP_XMIT_WIN, no OOB possible ***
RC=0