[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/pubspec.lock

@@ -193,6 +193,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.7"
+  device_info_plus:
+    dependency: "direct main"
+    description:
+      name: device_info_plus
+      sha256: a7fd703482b391a87d60b6061d04dfdeab07826b96f9abd8f5ed98068acc0074
+      url: "https://pub.dev"
+    source: hosted
+    version: "10.1.2"
+  device_info_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: device_info_plus_platform_interface
+      sha256: e1ea89119e34903dca74b883d0dd78eb762814f97fb6c76f35e9ff74d261a18f
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.3"
   drift:
     dependency: "direct main"
     description:
@@ -861,6 +877,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.3"
+  win32:
+    dependency: transitive
+    description:
+      name: win32
+      sha256: d7cb55e04cd34096cd3a79b3330245f54cb96a370a1c27adb3c84b917de8b08e
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.15.0"
+  win32_registry:
+    dependency: transitive
+    description:
+      name: win32_registry
+      sha256: "21ec76dfc731550fd3e2ce7a33a9ea90b828fdf19a5c3bcf556fa992cfa99852"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.5"
   xdg_directories:
     dependency: transitive
     description: