Add multi-iterable for zip iteration

[synapticvoid]

Summary

Extend for to support multiple bindings and multiple iterables with zip semantics.

Proposed Syntax

for a, b in xs, ys {
    print(a, b);
}

Semantics

• N bindings + N iterables, positionally matched
• Iteration stops at the shortest iterable
• Body scope includes all bound variables

Scope

• Parser/type-checker/runtime support for multi-binding + multi-iterable form
• Type inference per binding from corresponding iterable element type

Out of Scope

• Destructuring bindings
• Range/index sugar (tracked separately)

Acceptance Criteria

• for a, b in [1,2], [\"x\",\"y\"] { ... } works
• Stops at shortest iterable when lengths differ
• Type errors for binding/iterable arity mismatch