v3.20.0 (review fixes): the PoR decode uses the effective counter; the PWA fallback matches the server's seed

Two boundary flaws found reviewing v3.20.0:

- /api/send-ota decoded the PoR with the request's `cntr`, not the counter
  the packet was actually built with.  With the new seeding an omitted
  counter is the normal case, so a warning ENVELOPE's verdict - notably the
  live card's cntr_low PoR - could fail its RC/CC check and be lost (the
  success and the failure path are both fixed).  Tests pin the decode counter
  on both paths; the failure path also echoes the effective `cntr`.
- the PWA's effective counter (preview / verify / the older-server fallback)
  fell back to the preset's first keyset and then 1, while the server's empty
  seed builds counter 0 (no preset / kvn 0 / an unknown keyset).  A manual
  form or a keyless packet then produced a spurious "Verify vs pySim"
  MISMATCH and a preview that differed from the delivered packet.
  spPresetCounterValue()/spCntrEffective() now mirror _preset_counter_seed()
  exactly: the packet's matched keyset, else 0.
- the `OTA SEND: counter not tracked` line reflects the effective counter (a
  seeded value is logged too); docs/api.md records that the seed uses the
  packet's keyset whatever SPI1.b5b4 says.

Tests: 823 frontend + 737 Python; sw cache simple-v394.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v393';
+const CACHE = 'simple-v394';
 const URLS = [
   'index.html',
   'help.html',