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).

start.bat

@@ -15,6 +15,7 @@ set "HOST=127.0.0.1"
 set "PORT=4729"
 set "PCAP="
 set "DEBUG="
+rem Annotation rows (multi-spec form is sigrok-cli 0.7.x syntax; pin on upgrade)
 set "ANNOT=iso7816=apdus,iso7816=warnings"
 set "PROTOCOL=T=0"