quic: a bare FIN counts as sent only when ngtcp2 wrote it, not on any packet

[MDA2AV]

With the congestion window full and an ACK due, ngtcp2 answers a stream write with a packet holding only that ACK and reports the stream frame unwritten (*pdatalen = -1, as its docs say). PumpOut counted a bare FIN as sent whenever a packet came back, so that ACK swallowed it: the stream was done as far as the replay list knew, nothing retried the FIN, and the peer waited for an end that never came. A bare FIN now counts as sent only when the write reports 0 bytes consumed.

A streamed HTTP/3 response that calls CompleteAsync after its last flush ends with a bare FIN, on both stacks. h3x saw it as requests that never complete (13-43 per run of ~1.5M). A probe counted the FINs marked sent without being written - 22, against 22 hung requests and 22 streams that never closed - and ngtcp2's own log (a diagnostic build) showed each one: cwnd limited, then a 1-RTT packet holding only an ACK. h2load never showed them: in duration mode a stream still open at the end is not a failure.

Tests

quic: a FIN parked behind a full congestion window is still sent (new): the server fills its window on one stream and resets it, so the bare FIN it then sends on another is the only thing waiting. The client sends a packet after a lost one - an immediate ACK under RFC 9000 13.2.1 - so the parked FIN is the server's next write while the window is still full.

• main: FAIL 5 of 5 - the stream never ends
• this PR: pass 5 of 5

QuicTestClient gains Send(data, fin, lost), PumpUntil and per-stream byte and FIN tracking.

All suites pass: E2E 232, Unit 60, Chaos 47, Http 44, Tls 151 (the 7 kTLS tests skip without sudo), File 4.

Before and after

h3x, 16 conns × 32 streams, 1 KiB streamed responses, 6 s:

sample	main	this PR
Http3/ManagedStreamedBoth	13-43 never completed in each of 8 runs, every run waiting out 36 s	none in 8 runs, 6.1 s*
Http3/Nghttp3Response	23 and 41 in 2 runs	none in 2 runs

* one run lost a whole connection (32 requests) to a handshake timeout under load, which main shows as well - the kernel caps this machine's UDP buffer at 208 KiB.

h2load, 1 KiB, 16 conns × 32 streams, 4 interleaved rounds with a second build of main as control; within-round median vs main:

sample	this PR	control
Http3/ManagedStreamedBoth	-1.5%	+0.3%
Http3/Nghttp3Response	+1.7%	+1.9%
Http3/ManagedBuffered	-0.2%	+0.8%
Http3/Nghttp3Buffered	-2.0%	-2.9%