Hygiene: drop dead code, re-raise SystemError, resource+doc cleanup, add LICENSE

- pd.py: remove _lock_etu_from_edges (long-disabled) and the no-op
  while-True FIND START wrapper.
- pd.py: re-raise SystemError (libsigrokdecode's clean EOF signal) in the
  six 'except Exception: pass' guards so put()/wait() at end-of-capture
  exits cleanly instead of being swallowed.
- pd.py: fix _measure_clock_period docstring (averages, not robust-min);
  drop stale 'line 1633' comment; document the synthetic pcap UDP header
  (zero IP checksum / fixed loopback+ports are intentional).
- signal_diagnostic.py: add --max-samples cap + document whole-capture
  RAM cost; fix stale '--rst/--vcc=ignore' guidance (-> --no-rst/--no-vcc).
- tests: make test_valid_cla_interindustry_classes use subTest (was a
  no-op tuple expression).
- start.sh/start.bat: note the -A multi-spec form is sigrok-cli 0.7.x.
- Remove stale tests/baseline_new.txt (v1.1.3-era, ATR-included mode).
- Add top-level LICENSE (GPL-2.0-or-later).

tests/test_gsmtap.py

@@ -143,7 +143,8 @@ class TestValidateT0Apdu(unittest.TestCase):
         # messaging 0x0C, chaining 0x10, and the SM/structural 0x4x/0x5x.
         for cla in (0x01, 0x02, 0x03, 0x0C, 0x10, 0x14, 0x44, 0x50, 0x54):
             pkt = bytes([cla, 0xa4, 0x00, 0x04, 0x00, 0x90, 0x00])
-            self._valid(pkt), 'CLA=0x%02x' % cla
+            with self.subTest(cla=cla):
+                self._valid(pkt)
 
     def test_valid_cla_channel1_select_after_manage_channel(self):
         # The command that follows MANAGE CHANNEL (00 70 00 00 01) opening