Add break as value for loop expressions

[synapticvoid]

Summary

Allow break <expr> so loop expressions can return a value.

Motivation

Supports expression-oriented style while preserving explicit early-exit behavior.

Proposed Semantics

• Loop expression type is nullable/optional T?
• break value yields T
• Exhausting loop without value break yields null

Example

let first_even = for n in nums {
    if n % 2 == 0 { break n; }
}
// first_even: Int?

Scope

• Parser support for break with optional value
• Type checker support for loop result typing (T?) and break value consistency
• Runtime support for carrying break values out of loop execution

Out of Scope

• for...else
• Non-loop break targets

Acceptance Criteria

• Loop with break n type-checks as T?
• Loop without value break evaluates to null
• Type error when break values in same loop are inconsistent