main-0771de9

docs(perf): number the dufs UDP subsection and fix two cross-references

The Italian review's dufs chapter had an unnumbered subsection (the two data
planes compared on the real application) that the new recipe chapter cites as
§5.4, and the recipe chapter pointed at §4.3 for a figure that lives in §4.2.
Also corrects the carrier advice for the two `--udp` cases: on the direct path
`--carriers N` opens N independent QUIC connections, so 4-8 is right when the
reason is concurrency and 1 when the reason is packet loss — the single recipe
shown before implied 1 for both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BqfehFhjLR3Yj1sgaf5KCi