Add M2StartScene mission selection hub and controller

[jackandcarter]

Motivation

• Introduce an editor-built start hub for M2 so players can select missions from a galaxy map-style UI and set a home base location on the outer rim.
• Provide a lightweight controller to drive mission selection, previewing objectives, and starting mission flow instead of generating UI at runtime.
• Keep navigation consistent by adding a Return to Main Menu action wired into the hub UI.

Description

• Added a new scene Assets/M2StartScene.unity that contains an editor-built MissionSelectionCanvas with MissionListPanel, MissionDetailsPanel, and StartMissionPanel, plus an EventSystem and a HomeBaseLocation marker.
• Implemented Assets/M2StartScene/Scripts/M2StartSceneController.cs which defines mission data, wires mission button callbacks, updates mission preview fields, and exposes SelectMission, StartSelectedMission, and ReturnToMainMenu methods.
• Seeded three default missions (Outer Rim Recon, Supply Run, Derelict Recovery) and exposed references for UI text fields and buttons so designers can hook the editor UI to the controller.
• Added supporting folder and meta files under Assets/M2StartScene and Assets/M2StartScene/Scripts.

Testing

• No automated tests were executed for this change.
• The scene and controller are editor/UI changes that should be validated by opening M2StartScene in the Unity editor and verifying button wiring, mission preview updates, Start Mission navigation, and Return to Main Menu behavior.

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Codex Task