Function specialize 

pub fn specialize(program: &Program, shape: &Shape) -> Program

Specialize a program against a known document Shape. Current rewrites:

• OptField(k) → GetField(k) when shape guarantees the field exists (removes per-access None-check branch).
• KindCheck folded to a PushBool when shape at that path is known.

Returns a new Program with the same source/id.