[03-Developer] #218 Dev round 2 — AC-6 RAM 4GB gate + AC-10 docs cleanup

QA round 1 (commit 9a9eb2a) FAIL 시 누락된 두 AC 보강.

AC-6: device_info_plus 만으론 4GB 임계 측정 불가 (isLowRamDevice 는
~1GB 기준). MethodChannel `life_helper/device_caps` 신설 + MainActivity.kt
에서 ActivityManager.MemoryInfo.totalMem 노출. data/ai/device_capabilities.dart
는 DeviceCapabilities abstract + PlatformDeviceCapabilities + 4 GiB
임계. deviceMeetsAiRamProvider (FutureProvider<bool>, fail-closed).
SettingsScreen 토글 disabled + "RAM 부족" 안내 (RAM < 4GB).

AC-10: docs/reference/215-ai-frame-suggest.md 의 OQ-1/placeholder
6곳을 실 구현 표현으로 갱신. §8 알려진 제약 = AC-6 device gate +
AC-7 실 단말 E2E + F1 unload + #221 corpus 평가. §9 다음 단계 =
#219~#222 follow-up 목록. 신규 테스트 합계 41 / 전체 88 통과.

테스트: device_capabilities_test.dart 7 신규 (kAiMinRamBytes 동등,
null/0/3.9GB/4GB-1/4GB/8GB 경계). flutter analyze 무이슈, 전체 88 통과
(71 기존 + 10 gemma + 7 RAM gate).

Architect 설계서 §4 의 "RAM 4GB 차단 = AC-9 재활용" 문구는 사실 #215
미구현 사항이라 본 라운드에서 신규 추가했음을 README 에 명기.

Refs #218

app/android/app/src/main/kotlin/kr/cloud_handson/life_helper/MainActivity.kt

@@ -1,5 +1,36 @@
 package kr.cloud_handson.life_helper
 
+import android.app.ActivityManager
+import android.content.Context
 import io.flutter.embedding.android.FlutterActivity
+import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.plugin.common.MethodChannel
 
-class MainActivity : FlutterActivity()
+/// Hosts the `life_helper/device_caps` MethodChannel.
+///
+/// #218 AC-6: the AI feature requires ≥ 4GB RAM; getting an accurate total
+/// from Dart needs ActivityManager.MemoryInfo, which is Android-only — so we
+/// expose `totalMemoryBytes` as a platform method here.
+class MainActivity : FlutterActivity() {
+    private val deviceCapsChannel = "life_helper/device_caps"
+
+    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, deviceCapsChannel)
+            .setMethodCallHandler { call, result ->
+                when (call.method) {
+                    "totalMemoryBytes" -> {
+                        try {
+                            val am = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
+                            val info = ActivityManager.MemoryInfo()
+                            am.getMemoryInfo(info)
+                            result.success(info.totalMem)
+                        } catch (t: Throwable) {
+                            result.error("RAM_QUERY_FAILED", t.message, null)
+                        }
+                    }
+                    else -> result.notImplemented()
+                }
+            }
+    }
+}