Epic 3: Code Optimizations

## Epic 3a: Hook existing optimizations into pipeline

### Goal: wire the optimization infrastructure that's already built and tested.

- [ ] Hook constant propagation into pipeline (before codegen)
- [ ] Hook dead code elimination into pipeline
- [ ] Add pass manager + ordering tests (deterministic output)

## Epic 3b: New optimizations (build from scratch)

### Goal: add more passes; start with low-risk, high-win and ensure they preserve semantics.

- [ ] Copy propagation
- [ ] Common subexpression elimination (CSE)
- [ ] Peephole optimizations (algebraic simplification, redundant load/store, jump threading)
- [ ] Strength reduction (safe cases only; document assumptions)
- [ ] Devirtualization (static dispatch when receiver type is known)
- [ ] Void check elimination (skip isvoid checks when provably non-null)
- [ ] Loop-invariant code motion (LICM) - after CFG/SSA is stable
- [ ] Tiny method inlining (size threshold; watch for code bloat)
- [ ] Tail call optimization (recursive methods that return directly)