[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/lib/data/ai/device_capabilities.dart

@@ -0,0 +1,58 @@
+import 'dart:io';
+
+import 'package:flutter/services.dart';
+
+/// Minimum RAM (bytes) required for on-device Gemma 4 E2B inference.
+///
+/// 4 GiB matches Planner AC-6 of #218. The Gemma 4 E2B weights alone are
+/// ~2.4GB; adding KV-cache + Flutter runtime + OS headroom puts us at 4GB
+/// total as the practical floor below which AC-7 cold-start budgets fail.
+const int kAiMinRamBytes = 4 * 1024 * 1024 * 1024;
+
+/// Abstraction over the platform-channel RAM query, so tests can inject a
+/// fake without touching MethodChannel.
+abstract class DeviceCapabilities {
+  /// Returns total physical RAM in bytes, or `null` if unknown / unsupported
+  /// (non-Android host, channel error). Callers must treat `null` as "do
+  /// not enable the AI gate" (fail-closed).
+  Future<int?> totalRamBytes();
+
+  /// Convenience: `true` iff [totalRamBytes] returns ≥ [kAiMinRamBytes].
+  /// `null` from [totalRamBytes] → `false` (fail-closed).
+  Future<bool> meetsAiMinRam() async {
+    final bytes = await totalRamBytes();
+    if (bytes == null) return false;
+    return bytes >= kAiMinRamBytes;
+  }
+}
+
+/// Real implementation. Calls `MainActivity.kt` over a MethodChannel.
+class PlatformDeviceCapabilities implements DeviceCapabilities {
+  PlatformDeviceCapabilities({MethodChannel? channel})
+      : _channel = channel ??
+            const MethodChannel('life_helper/device_caps');
+
+  final MethodChannel _channel;
+
+  @override
+  Future<int?> totalRamBytes() async {
+    // Channel is Android-only — return null on iOS/host tests rather than
+    // throwing MissingPluginException.
+    if (!Platform.isAndroid) return null;
+    try {
+      final v = await _channel.invokeMethod<int>('totalMemoryBytes');
+      return v;
+    } on PlatformException {
+      return null;
+    } on MissingPluginException {
+      return null;
+    }
+  }
+
+  @override
+  Future<bool> meetsAiMinRam() async {
+    final bytes = await totalRamBytes();
+    if (bytes == null) return false;
+    return bytes >= kAiMinRamBytes;
+  }
+}