docs: improve Context7 docs for Bluetooth, BroadcastReceiver, and general patterns

Address feedback on documentation gaps:

- bluetooth.md: Add complete BluetoothAdapter acquisition flow,
  startDiscovery JNI signature ("()Z") and CallBooleanMethod usage,
  end-to-end classic discovery with BroadcastReceiver registration,
  BLE scanning with GoAbstractDispatch adapter pattern, GATT
  client/server sections, expanded constants reference.

- broadcast-receiver.md: Rewrite with registerReceiver as primary
  approach. Complete GoBroadcastReceiver.java adapter, Go handler
  registration, IntentFilter creation, ctx.RegisterReceiver2() call,
  and cleanup. No longer deflects to alternative APIs.

- general-patterns.md: New guide for Android APIs not covered by
  specific examples. Covers system services, constructors, static
  methods, interface vs abstract class callbacks, reference management,
  HandlerThread, JNI signatures, value constructors, common pitfalls.

- context7.json: Add rules for abstract class pattern, NewProxy
  limitation, BooleanValue type, and pointer to general-patterns.md.

Author: xaionaro@dx.center

context7.json

@@ -28,6 +28,10 @@
     "Generated packages cache method IDs via ensureInit(); prefer typed wrappers over raw JNI calls",
     "Java exceptions are automatically converted to Go errors by all Call*Method functions",
     "For callbacks on Java interfaces, use env.NewProxy() to create a java.lang.reflect.Proxy",
-    "Create a HandlerThread with its own Looper when registering listener callbacks"
+    "For callbacks on abstract classes (BroadcastReceiver, ScanCallback, etc.), use a Java adapter class with GoAbstractDispatch + jni.RegisterProxyHandler on the Go side",
+    "env.NewProxy() only works for Java interfaces, NOT abstract classes -- see broadcast-receiver.md for the abstract class pattern",
+    "Create a HandlerThread with its own Looper when registering listener callbacks",
+    "CallBooleanMethod returns uint8; convert to Go bool via resultRaw != 0. BooleanValue() takes uint8, not Go bool",
+    "See general-patterns.md for guidance on Android APIs not covered by specific examples"
   ]
 }