feat(audit): a run manifest should answer questions about its own run - #55

Review finding #1. ScenarioManifest recorded `vendor` and `duration`;
RunManifest recorded neither, and nothing recorded the rate actually in force or
what the socket really did. A manifest that cannot say which profile rendered it
is a weaker audit record than safety rule 5 implies, and the web layer had to
paper over the gap.

Four fields, all defaulted so manifests written before them still load, which is
the convention `pace` and `speed` already set:

- `vendor`      which dialect rendered these events
- `duration`    the --duration asked for, verbatim
- `rate`        the events-per-second ceiling in force
- `send_stats`  sends, bytes, errors, oversize

`send_stats` is None rather than a row of zeroes when a run had no collector,
because zeroes read like a send that delivered nothing. `event_count` counts
events rendered; `send_stats["sends"]` counts datagrams the kernel accepted, and
the two differing is the interesting case rather than an inconsistency.

Captured on every exit path, so a run that failed part-way still records what its
socket managed before it died.

Read with getattr rather than `emitter.stats`, because the emitter is an
injection point and several test doubles implement only send/close. Requiring the
attribute would make this record cost every future fake a field it does not
otherwise need. Four existing test doubles proved the point immediately.

875 py. black, ruff, mypy clean.