Follow-up tasks

Work found while fixing something else and left out of that change so it stayed one logical fix. Each entry stands alone and can be handed to an agent as is. Delete an entry when its fix lands.

The SIPp C++ source is at ../../cprojects/sipp/src (relative to the repo root). Read it to confirm the behavior, and never copy it (GPL). Every task follows the same loop:

1. Write a failing test first, preferably an interop test against real sipp. jumps_and_message_indices_skip_labels_like_real_sipp in tests/interop.rs shows the UAC-against-a-UDP-sink pattern.

2. Fix it, then update docs/SIPP_COMPAT.md §6 (mark the open item fixed) and the Unreleased section of CHANGELOG.md.

3. Run the gates:

   cargo fmt --all -- --check
   cargo clippy --workspace --all-targets -- -D warnings
   cargo test --workspace
   cargo deny check
   SIPP_BIN=../../cprojects/sipp/sipp cargo test -p sipr --test interop
   

4. Commit per docs/CONVENTIONS.md, with the scope given below.

No open entries.