[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/state/ai_providers.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../data/ai/device_capabilities.dart';
 import '../data/ai/llm_service.dart';
 import '../data/ai/model_lifecycle.dart';
 import '../data/db/app_database.dart' as drift;
@@ -22,6 +23,19 @@ const _kModelUrl =
 const _kModelSha256 =
     '181938105e0eefd105961417e8da75903eacda102c4fce9ce90f50b97139a63c';
 
+/// #218 AC-6: device-capability gate. RAM < 4GB → AI feature disabled.
+/// Default implementation calls the `life_helper/device_caps` MethodChannel
+/// (Android). Override in tests with a `_FakeDeviceCapabilities`.
+final deviceCapabilitiesProvider = Provider<DeviceCapabilities>((ref) {
+  return PlatformDeviceCapabilities();
+});
+
+/// `true` iff the device has ≥ 4GB RAM. Default `false` (fail-closed) while
+/// the platform call is in flight or on unsupported hosts (iOS / test).
+final deviceMeetsAiRamProvider = FutureProvider<bool>((ref) async {
+  return ref.watch(deviceCapabilitiesProvider).meetsAiMinRam();
+});
+
 final modelLifecycleProvider = Provider<ModelLifecycle>((ref) {
   return ModelLifecycle(
     meta: ref.watch(metaDaoProvider),