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

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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.