Phase 2: Add step-by-step opcode execution trace

[polydeuces32]

Goal

Add a step-through execution mode that shows exactly how each opcode changes the stack.

Why

This is the most valuable upgrade for learning. It turns the playground from a simple runner into an actual teaching tool.

Requirements

• Add executionTrace output from the engine
• Each trace item should include:
  • token index
  • token name
  • stack before
  • stack after
  • error state if any
• Add UI buttons:
  • Run All
  • Step Forward
  • Reset Trace
• Highlight the currently executing opcode
• Show stack before/after for each step

Acceptance Criteria

• User can run a script one opcode at a time
• Unknown tokens fail clearly
• UI never uses unsafe innerHTML for user-provided values
• Existing run behavior still works