v3.18.4: run-log review follow-ups (one ENVELOPE line per event step)

The review of v3.18.3 found the event steps logging twice: the shared sender
writes its generic `ENVELOPE(Event Download): type=... data=...` /
`ENVELOPE SW: ...` pair, and the runner's step-annotated line repeated the
same data.  `_send_event_download` gained `log=True`; the runner's envelope
and event branches pass `log=False`, so an event step logs exactly one line -
the annotated one, which already carries the step number, the event name,
the effective source and the response SW.  All other callers keep their
output unchanged (netsim, the interactive event form, the BIP/timer paths).

The review's test-pinning gaps are closed:
- the shared sender's per-segment answer line
  (`OTA SEND: ENVELOPE i/N -> <SW>`) is asserted in the multi-segment test;
- the scp80 step's report `sent` C-APDU/SP is asserted in the run-log test;
- the single-ENVELOPE-line behaviour (and the source override) is asserted
  in the event-action test.

Tests: 816 frontend + 718 Python; sw cache simple-v386.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v385';
+const CACHE = 'simple-v386';
 const URLS = [
   'index.html',
   'help.html',