Search over candidate signature instructions and keep the best program.

InstructionSearch is the small, explicit optimizer underneath instruction-only workflows such as Imp.Optimizer.SignatureOptimizer and coordinate prompt optimization. It evaluates each proposed instruction on the dev set, evaluates the original program as a baseline, and attaches an optimizer report to the selected program. The original program wins equal-score ties so a rewrite must demonstrate an actual validation gain.

Failed candidates are recorded in the report instead of aborting the whole compile. If every evaluation fails, compile/6 returns the original program with a diagnostic report.