Decode standard PPS sub-types + RST CLK extension → ATR data rate

- capture.py: map GSMTAP PPS request/response (0x02/0x03) to 'pps',
  matching the pcap-import mapping (was decoded as TPDU live)
- decode.py: parse the optional RST CLK extension (sigrok v1.9.0+,
  payload flag bit 0); ATR gains clk_hz/data_rate/etu_us derived from
  TA1 Fi/Di (data_rate = clk_hz × D / F)
- database.py: CLK state is consumed by the next ATR (no stale rate
  for an ATR without preceding RST); a late-arriving RST measurement
  is backfilled onto the ATR it belongs to; replay context carries
  the CLK across pagination/live polling
- PWA: ATR detail shows CLK/Data rate/ETU; RST event detail shows CLK
- tests: PPS mapping, CLK parse with/without flag, rate math,
  DB consume + backfill round-trips
- README/RUS updated
- Bump 1.22.18, SW cache v49->v50

tests/test_capture.py

@@ -19,7 +19,8 @@ class TestGsmtapSubtypeMapping(unittest.TestCase):
     def test_standard_subtypes(self):
         self.assertEqual(gsmtap_msg_type(0x00), 'tpdu')
         self.assertEqual(gsmtap_msg_type(0x01), 'atr')
-        self.assertEqual(gsmtap_msg_type(0x02), 'tpdu')  # PPS combined
+        self.assertEqual(gsmtap_msg_type(0x02), 'pps')  # PPS request
+        self.assertEqual(gsmtap_msg_type(0x03), 'pps')  # PPS response
 
 
 class _FakeBackend: