Add AST-to-bytecode compiler for expression core with parity tests

[synapticvoid]

Summary

Compile the existing expression/core statement subset from AST into bytecode and validate parity against the current interpreter.

Scope (MVP)

• Lower from typed AST to bytecode for:
  • literals
  • unary/binary expressions
  • var declaration/get/set
  • block expressions
  • if expressions (with jumps)
  • expression statements
• Wire compile+run path behind a VM entry point
• Add parity test harness:
  • run same source via current interpreter and VM
  • compare value/output for supported subset

Out of Scope

• Full function declaration/calls if too large for first slice (can be follow-up)
• Struct/enum/error unions/try/catch lowering
• import system and module linking

Acceptance Criteria

• Supported subset programs compile and run on VM
• Parity tests pass against interpreter for supported subset
• Compiler emits useful errors for unsupported AST nodes