feat: detect screen SafeArea ownership for Flutter guidance

[PacteraSunChao]

Summary

• Detect when a screen must own the runtime top safe-area inset (no App Bar present to absorb it).
• Expose ScreenSafeAreaInfo on screen layout analysis output.
• Generate Flutter Scaffold guidance that wraps the body in SafeArea only for unhandled edges, and routes footer sections to bottomNavigationBar.

Motivation

Design frames often omit system insets. Without explicit safe-area ownership, agents either hardcode design insets into component heights or wrap the whole screen blindly. This change records whether the screen itself must handle the top inset, and emits matching Flutter guidance.

Changes

• Types: add ScreenSafeAreaInfo and attach optional safeArea to ScreenLayoutInfo.
• Extractor: detectScreenSafeArea() returns { top: { required: true, reason: 'no-app-bar' } } when no App Bar is detected.
• Reports / guidance: print Top Safe Area evidence; clarify that insets belong to screen composition; wrap body in SafeArea with top/bottom disabled when AppBar/footer already occupy those edges.

Test plan

• npm run build succeeds
• Analyze a screen without App Bar → report includes Top Safe Area: Required at runtime
• Analyze a screen with App Bar → no top SafeArea requirement; guidance uses SafeArea(top: false, ...) when only bottom needs protection
• Footer section appears under bottomNavigationBar, not inside body children
• Spot-check that Logger-only changes are not included