Define MoE extraction traits inside this crate

[rmems]

Define MoE extraction traits inside this crate. Make it completely zero-dependency and ready for wider use.

Acceptance Criteria:

• MoE extraction traits are defined
• Crate remains zero-dependency
• Ready for potential publishing

Affected files:

• engram-parser/src/moe/extract.rs
• engram-parser/src/lib.rs