fix: restore mtu probe promotion

[marcus-pousette-hp]

This fixes MTU probe promotion after 1302a45, which replaced assert(wanted_size > pkt->size) with an early return. That condition is the normal in-band probe case: the packet is smaller than the target MTU and should be padded up to the probe size.

The fix removes that inverted return and rejects invalid padding after computing it. Negative padding means the packet is already larger than the target probe size; padding above 255 cannot fit in the header field.

I added a regression test that transfers 8 MiB over loopback and asserts that successful probes promote the sender from UDX_MTU_BASE to UDX_MTU_MAX.

I also tested this through udx-native@1.20.6 against a remote blind relay. Both peers ran locally and were forced through the remote relay; the remote host was only the relay. Each run transferred 8 MiB.

Build	Passes	Avg MTU	RTOs	Retransmits	Byte multiplier	Throughput
before	5/5	1200 / 1200	4	20,238	1.46x	0.636 MiB/s
after	5/5	~1443 / ~1437	3	14,120	1.38x	0.673 MiB/s

Verified with ctest --test-dir build-debug --output-on-failure: 30/30 passed, with the existing perf test disabled.

Co-written with AI