Add PS_Editor plugin Phase 1: GameMode + Camera navigation

Runtime editor plugin with UE viewport-style camera controls:
- ZQSD/Arrows movement (always active), E/A for up/down
- LMB: pan (horizontal move + yaw), RMB: free fly look
- Alt+LMB / MMB: orbit around focal point, scroll: zoom
- HUD with controls help, cursor auto-hide during camera ops

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

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+# PS_ProserveEditor - Project Guidelines
+
+## Project Overview
+Unreal Engine 5.5 project with runtime editor plugin (PS_Editor). The plugin enables runtime scene editing in packaged builds: object placement, property editing, scenario definition, splines, lighting, and JSON scene save/load.
+
+## Architecture
+- **Plugin**: PS_Editor (runtime plugin, must work in packaged builds)
+- **UI**: UMG driven by C++
+- **Gizmos**: InteractiveToolsFramework (UE5 built-in)
+- **Serialization**: JSON via Json/JsonUtilities modules
+- **Input**: Enhanced Input system
+
+## Naming Conventions
+- All plugin files and classes use the `PS_Editor_` prefix (e.g., `PS_Editor_GameMode`, `PS_Editor_Pawn`)
+- Code and comments in English
+- Standard UE5 prefixes: `A` for Actors, `U` for UObjects, `F` for structs, `E` for enums, `I` for interfaces
+
+## Code Quality
+- Prefer correct, complete implementations over minimal ones.
+- Use appropriate data structures and algorithms — don't brute-force what has a known better solution.
+- When fixing a bug, fix the root cause, not the symptom.
+- If something requires error handling or validation to work reliably, include it without asking.
+
+## Build
+- Engine: Unreal Engine 5.5
+- Plugin location: `Unreal/Plugins/PS_Editor/`
+- Game module: `PS_ProserveEditor` (in `Unreal/Source/PS_ProserveEditor/`)
+- CLI build command (Editor, Development):
+  ```
+  "<UE_INSTALL>/Engine/Build/BatchFiles/Build.bat" PS_ProserveEditorEditor Win64 Development -Project="<REPO>/Unreal/PS_ProserveEditor.uproject" -WaitMutex -FromMsBuild
+  ```
+  UE install path can be found via registry: `HKLM\SOFTWARE\EpicGames\Unreal Engine\5.5` > `InstalledDirectory`
+
+## Project Memory
+Shared project memory is stored in `.claude/memory/` (versioned in the repository). When starting a new session on any machine, read these files to restore context:
+
+- `MEMORY.md` — Index of all memory files with one-line summaries
+- `project_ps_editor.md` — Project scope, technical decisions, current phase
+- `feedback_naming.md` — Naming conventions and user preferences
+- `reference_build.md` — Full build command and UE install path details