Spec: Effects system sketch (IO and purity)

[JROChub]

Context

• We want a language with strong purity guarantees, but there is no effects model in the spec yet.

Goal

• Provide a minimal effects system sketch that fits the vision and guides future implementation.

Proposal

• Add a spec section describing:
  • Pure vs effectful functions.
  • IO as an effect (and how effects are tracked).
  • Syntax and typing expectations.
  • Possible effect handlers or capabilities model.
• Include a few small examples.

Acceptance criteria

• A clear, minimal effects model is documented with examples.
• The section notes what is intentionally deferred or undecided.