Format log context only when a line is logged

[WYCLIFF001]

The session loop builds format!("local {{ seq: {seq}, ack: {ack} }}") and tcp_header_fmt(tcp_header) for every packet, and poll_write builds another string for every held write, whatever the log level. At the default level none of them is printed, so every packet pays for allocations and formatting that go unused. In a profile of a 600 Mbit/s tunnel these showed up as core::fmt plus the matching malloc/free on every packet.

• l_info is now a small LocalInfo(seq, ack) value that implements Display, so it formats only inside a log line that is actually emitted. The six call sites are unchanged apart from the constructor.
• The per-packet header description is built only under log::log_enabled!(log::Level::Trace).
• The held-write message is formatted inline in its trace!.

Log output is identical at every level.

cargo fmt --check, cargo clippy --all-targets (no warnings) and cargo test pass.

🤖 Generated with Claude Code

[SajjadPourali]

Reviewed the logging changes. The expensive header formatting and contextual string construction are deferred until the matching log level is enabled; packet handling is unchanged. CI passes.