Function compile_program 

pub fn compile_program(
    program: &Program,
) -> Result<CompiledProgram, ValidationError>

Compile a validated program into the most efficient execution body.

Selection order is intentionally greedy:

1. internal specialized plan
2. external typed host plan
3. compiled pipeline IR
4. tree fallback

This design gives large speedups for common production shapes while keeping full coverage for arbitrary valid programs.