Closes the CDD design phase. Records:
- §E.1 contract inventory (every file exists, compileall clean).
- Targeted pytest pass: 604 passed, 1 skipped, 10 xfailed
(all xfails are `xfail(strict=True)` with reason= pointing to the
impl step that flips them; carry-overs, not flakes).
- Strict mypy over decnet/ttp + decnet/cli/ttp.py +
decnet/web/router/ttp + decnet/web/db/sqlmodel_repo/ttp.py: clean.
- Stranger-readability spot check on tests/ttp/: no doc bugs.
Notes the three pre-E.4 wiring fixes (E.3.18a/b/c) and the E.4
backfill CLI / DEBT entries that landed in this series.