Add M2 Foundations docs and expand Architecture/README

[jackandcarter]

Motivation

• Centralize the Maelstrom2 (M2) design into a single reference to guide mission development and documentation.
• Clarify that M2 uses designed, goal-driven missions rather than looping arena survival.
• Expose the M2StartScene hub as the pre-mission decision point and link scene flow to architecture docs.
• Provide explicit mission types and core design pillars to guide level and systems implementation.

Description

• Added a new documentation file Assets/Docs/M2Foundations.md containing the full M2 foundations text including Overview, Core Design Pillars, Mission Types, Scene Flow, Current Status, and Future Updates.
• Expanded Assets/Docs/Architecture.md with a new "Maelstrom2 (M2) Mode" section that includes the M2StartScene hub reference, Core Design Pillars, and Mission Types.
• Updated README.md documentation list to include the new M2 Foundations entry linking to Assets/Docs/M2Foundations.md.
• Ensured cross-references between M2Foundations.md and Architecture.md to keep scene flow and hub information discoverable.

Testing

• This is a documentation-only change, so no code tests were executed.
• No Unity Test Runner or play-mode tests were run as part of this update.
• Documentation files were added/updated and saved without generating build output.
• Markdown links were verified in-file to point to Architecture.md and Assets/Docs/M2Foundations.md.

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Codex Task