Implement the optimiser: constant folding, dead code, comparison-branch peephole

[kameronbrooks]

Plan item #24. optimize() in compiler.ts is an empty stub.

Obvious first passes, in rough order of value:

• Constant folding of scalar BinaryOp expressions (the compile-time-value machinery was made trustworthy in the Compound assignment %= does not parse #7 fix).
• Dead-code removal after OP_JUMP / OP_TERM.
• OP_NOOP elimination with label fixup (blocked on the scoped-labels refactor, plan #25 — the padding OP_NOOPs exist to flush pending labels).
• comparison + OP_BRANCH_FALSE → single comparison-branch opcode (the 18 comparison-branch opcodes exist and their operand order was fixed in #21; this peephole is what would finally emit them).

Every pass must be mirrored or verified on the WebAssembly backend via the differential suite.