Compiler optimizer v0 — semantics-preserving transformations with witnesses

[juv4uk]

Goal

Add the first optimization pass only after compiled execution has a parity witness.

An optimization is admitted only as a transformation with an explicit applicability condition and executable evidence that observable semantics are preserved. The backend must not silently 'know better' than Lisp.

Required work

• Choose one narrow optimization whose safety conditions can be stated precisely.
• Represent the transformation and its witness/provenance in inspectable data.
• Preserve exact integers/rationals exactly; never introduce float approximation.
• Preserve observable evaluation/error behavior under the stated preconditions.
• Keep the transformation rule at the Lisp/semantic layer where practical; Rust may perform mechanical execution of an already-admitted transform but not define its meaning.

Acceptance evidence

For a corpus containing both eligible and deliberately ineligible cases:

• optimized compiled result == unoptimized compiled result == meta/native oracle;
• ineligible cases are left unchanged or rejected, never guessed;
• the test can identify which transformation fired and why.

Dependencies

• Compiler authority — make semantic ownership non-negotiable #66
• Compiler oracle corpus — freeze observable semantics before lowering #67
• Compiler IR v0 — provenance-bearing semantics-neutral lowering data #68
• Compiler first witness — execute compiled code with exact meta/native parity #69

Non-goals

• No optimizer grab-bag.
• No optimization accepted on benchmark speed alone.