Implement for loops after model basics

[itsfuad]

Goal

Add for support after array/slice/optional basics are stable.

Scope

• Start with arrays and slices.
• Add iterator protocol only after next / optional return semantics are usable.
• Avoid dynamic interface dispatch for normal array/slice loops.
• Allow dynamic dispatch only when source type is already interface/dynamic.

Validation

• Parser/typechecker/HIR/MIR/backend tests.
• Runnable x_test for for i in xs, for i, v in xs, and infinite for.