docs: rewrite general-patterns.md to lead with typed wrappers

Restructure the guide to reflect the project's goal of idiomatic,
strongly typed Go code:

- Lead with typed wrappers as the default (NewManager, typed methods,
  data class accessors)
- Add section on named constants with "never magic numbers" guidance
- Show return type mapping (Java -> Go) with automatic conversions
- Move raw JNI to a short fallback section at the end
- Remove magic constant 512 from bluetooth.md GATT example
- Fix constant comparisons to use named constants throughout

Author: xaionaro@dx.center

docs/bluetooth.md

@@ -543,8 +543,10 @@ func gattClient(vm *jni.VM, ctx *app.Context, device *bluetooth.Device) error {
     // Enable notifications for a characteristic
     ok, err = gatt.SetCharacteristicNotification(characteristicObj, true)
 
-    // Request MTU change (triggers onMtuChanged callback)
-    ok, err = gatt.RequestMtu(512)
+    // Request MTU change (triggers onMtuChanged callback).
+    // 517 is the maximum ATT MTU for BLE (Android caps at 517).
+    const maxBleMtu = 517
+    ok, err = gatt.RequestMtu(int32(maxBleMtu))
 
     // Read remote RSSI (triggers onReadRemoteRssi callback)
     ok, err = gatt.ReadRemoteRssi()