Add generic SAE DTC database (1409 codes) with profile-priority fallback

Compiled by the generic-dtc-db workflow (P0/U0/C0/B0 standard codes, system
tags + no-start flags). Lives in profiles/_data/generic-dtcs.json (bundled with
profiles/, not listed as a vehicle profile). DtcDatabase.get now falls back:
profile code -> generic code -> unknown, so any standard code resolves to a
description while vehicle profiles still override (e.g. P0148 keeps the 6.0 text).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

obdcore/registry.py

@@ -81,12 +81,35 @@ class PidRegistry:
         return list(self.presets.keys())
 
 
+_GENERIC = None
+
+
+def _generic_dtcs():
+    """Lazy-load the bundled generic SAE DTC database (code -> Dtc)."""
+    global _GENERIC
+    if _GENERIC is None:
+        _GENERIC = {}
+        try:
+            import json
+            import os
+            from .profile import profiles_dir
+            path = os.path.join(profiles_dir(), "_data", "generic-dtcs.json")
+            for d in json.load(open(path)).get("dtcs", []):
+                _GENERIC[d["code"]] = Dtc(code=d["code"], desc=d.get("desc", ""),
+                                          system=d.get("system", "powertrain"),
+                                          no_start=d.get("no_start", False))
+        except Exception:
+            pass
+    return _GENERIC
+
+
 class DtcDatabase:
     def __init__(self, profile):
-        self._db = {d.code: d for d in profile.dtcs}
+        self._db = {d.code: d for d in profile.dtcs}   # profile codes take priority
 
     def get(self, code):
-        return self._db.get(code) or Dtc(code, "(unknown - look up this code)")
+        return (self._db.get(code) or _generic_dtcs().get(code)
+                or Dtc(code, "(unknown - look up this code)"))
 
     def all(self):
         return list(self._db.values())