v3.18.0: the semantic event action for test scripts

The other-applet batch: a test script can send the common event downloads
semantically instead of raw hex.

- events.py: server-side mirrors of the PWA's event forms for Location status
  (0x03), Access technology change (0x0B), Network rejection (0x12 - the
  LA/RA/TA object selected by the registration group, the extended EMM cause
  and the extended information object) and Data connection status change
  (0x1D - status/type/TI plus the optional cause, host-clock 8.39 date-time,
  location information, access technology, APN and PDP/PDN/PDU type).  Fields
  are validated before the run (a bad value is a script error, not a step
  failure); `src` overrides the device-identities source (default terminal,
  matching the Phone forms).
- The engine gains the `event` action kind; the runner builds and sends it.
  The PWA's action-kind list stays in sync (drift-guarded - the guard itself
  caught its own regex bug on the first run).
- Byte parity: the vectors are generated from the PWA's own builders and
  pinned in tests/test_events.py; the frontend suites assert the same values.
  PLMN and SCTS helpers are mirrored too.
- The Phone tab's event form shows the built "Event data (hex)" (updated as
  the fields change), so every event without a builder can be used through
  the raw `envelope` action.
- The test-step editor offers the event select + a JSON fields textarea;
  malformed JSON refuses the save with a clear message.
- docs/test-scripts.md (the semantic-event table, an event example, the
  format stamp), docs/api.md and help EN/RU updated.

Tests: 812 frontend + 717 Python; sw cache simple-v381.

frontend/tests/event_forms.test.js

@@ -296,3 +296,28 @@ test('the single-envelope budget is enforced by the size helper', () => {
 	assert.ok(!eventFitsOneEnvelope('AB'.repeat(245)));
 	assert.ok(eventFitsOneEnvelope(null));
 });
+
+test('the event form previews the bytes the send would deliver', () => {
+	// the preview builds from the visible fields - the same bytes the send
+	// uses, so a raw-envelope test step can be authored by copying it
+	const els = {
+		'ev-f-status': { value: '0' },
+		'ev-f-mcc': { value: '250' },
+		'ev-f-mnc': { value: '01' },
+		'ev-f-lac': { value: '00FF' },
+		'ev-f-cell': { value: '0001' },
+		'event-send-preview': { value: '' },
+	};
+	globalThis.document = { getElementById: id => els[id] || null };
+	globalThis._eventFormType = 0x03;
+	pysimEventPreview();
+	assert.strictEqual(els['event-send-preview'].value, '9B0100930752F01000FF0001');
+	// a hidden field is left out of the build
+	els['ev-f-mcc'].closest = () => ({ style: { display: 'none' } });
+	pysimEventPreview();
+	assert.strictEqual(els['event-send-preview'].value, '9B0100');
+	// an event without a form leaves the preview empty
+	globalThis._eventFormType = 0x99;
+	pysimEventPreview();
+	assert.strictEqual(els['event-send-preview'].value, '');
+});