v3.19.0 (docs/engine): test-script guide audit; sms da accepts a leading +

An audit of docs/test-scripts.md against the engine (testscript.py,
events.py, the runner) found a handful of doc-level gaps, now fixed:

- the scp80 param defaults (tar = the preset's ISD role TAR, spi1 = that
  TAR's MSL, spi2 = 01, kvn = the first keyset);
- the counter rule as implemented: the run advances/persists the counter
  after every answered scp80 step when SPI1 requests a counter check
  (refused packets move the local value forward too - it only moves up);
- respond is optional (an omitted object answers ok/0x00);
- the run-log section: the exact READ BINARY/READ RECORD/UPDATE BINARY,
  SCP80 WRAPPED=/SP= and 'TEST-RUN step N failed:' lines;
- Known limitations: an expanded-format 61xx has no in-packet GET RESPONSE
  (TS 102 226 5.2.1.1) - send the GET RESPONSE as a follow-up scp80 step;
- sms check da: a leading + and whitespace are ignored.  The engine now
  drops the + (the TPDU carries the international flag in the dialling
  type, so '+79...' could otherwise never match the parsed digits);
  docs/api.md and the local AGENTS note it.

Tests: 821 frontend + 726 Python.

pysim_simple_server/testscript.py

@@ -467,7 +467,11 @@ def _normalise_content_check(c, default_level):
         da = c.get('da')
         if da is not None and str(da).strip():
             d = re.sub(r'\s', '', str(da))
-            if not re.fullmatch(r'[0-9*#+]{1,20}', d):
+            # The international flag lives in the TPDU's type-of-number, not
+            # in the digits: a leading '+' is accepted and dropped.
+            if d.startswith('+'):
+                d = d[1:]
+            if not re.fullmatch(r'[0-9*#]{1,20}', d):
                 raise ScriptError('sms check: da must be up to 20 digits')
             out['da'] = d
         for key in ('pid', 'dcs'):