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/memory/project_ps_editor.md

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+---
+name: PS_Editor plugin project
+description: Runtime editor plugin for UE 5.5 - object placement, scene editing, JSON save/load in packaged builds
+type: project
+---
+
+PS_Editor is a runtime editor plugin for Unreal Engine 5.5 that will be integrated into the PROSERVE project.
+
+**Why:** Enable runtime scene editing in packaged builds - place objects, configure properties, define scenarios, manipulate splines, modify lighting, and save/load scenes as JSON.
+
+**How to apply:** All development is incremental, phase by phase. Phase 1 = GameMode + Camera (completed). Future phases: object spawning/transform, JSON serialization, splines, lighting, AI characters.
+
+Key decisions:
+- UI: UMG driven by C++ (not Blueprints)
+- Gizmos: InteractiveToolsFramework (UE5 built-in)
+- Serialization: JSON via Json/JsonUtilities modules
+- Naming: PS_Editor_ prefix on all files/classes
+- Plugin path: Unreal/Plugins/PS_Editor/