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.

docs/api.md

@@ -702,7 +702,8 @@ SHORT MESSAGE carrying the PoR of the
 preceding `scp80` step - PoR-in-submit, SPI2 bit `20`: `status` (the response
 status name, `ok` = `por_ok`), `sw` (the R-APDU status word, exact/mask) and
 `data` (the R-APDU response data, exact/mask)), `sms` (the SEND SHORT MESSAGE
-TPDU: `da` digits, `pid`/`dcs`/`ud` hex exact/mask, `udl` decimal; SMS-SUBMIT
+TPDU: `da` digits with whitespace skipped and a leading `+` ignored,
+`pid`/`dcs`/`ud` hex exact/mask, `udl` decimal; SMS-SUBMIT
 only) or `files` (the File List of a
 REFRESH, TS 102 223 8.18; `files` is a list of path hex strings, matched
 order-insensitively); `respond` is the TERMINAL RESPONSE in the TS 102 223