h2: RFC 9218 incremental flag unused in priority scheduling

[FlorentinDUBOIS]

Summary

The Prioriser struct in lib/src/protocol/mux/h2.rs stores per-stream (urgency, incremental) tuples per RFC 9218, but the incremental flag is discarded during stream scheduling in write_streams():

priorities.sort_by(|a, b| {
    let (ua, _) = self.prioriser.get(a);  // incremental ignored
    let (ub, _) = self.prioriser.get(b);
    ua.cmp(&ub).then_with(|| a.cmp(b))
});

Per RFC 9218 §4, incremental=true streams within the same urgency level should use round-robin scheduling, while incremental=false streams should be served sequentially (complete one before starting the next).

Current behavior

All streams within the same urgency level are served in stream-ID order (FIFO), regardless of the incremental flag.

Expected behavior

• incremental=false (default): serve streams sequentially within urgency level
• incremental=true: interleave DATA frames across streams within urgency level

Priority

Low — urgency-based ordering already provides meaningful prioritization for the common case.