Extend minimal builder contract to require empty constructor #308

Sub-task of #301.

Goal

Extend the contract check in BuilderScopeResolver for referenced builders to require a public no-arg constructor B() (until the sibling task adds static factories) in addition to the existing B(T) + build() checks.

Why

The nested-builder consumer emits new FooBuilder() when the field is unset, but the contract today only verifies ctor(T) + build() — a hand-written builder without a no-arg ctor passes and produces uncompilable generated code.

Breaking change

Referenced builders that satisfy today's contract but lack B() stop qualifying; their fields degrade to plain setters (correct output, but observable).