Phase 136: VM Core Variables - Identifier Modernization

Renamed 5 core variables in VirtualMachine::execute() main loop:
- `cl` → `currentClosure` (~10 uses)
- `k` → `constants` (~15 uses)
- `base` → `stackFrameBase` (~30 uses)
- `pc` → `programCounter` (~35 uses)
- `trap` → `hooksEnabled` (~13 uses)

**Total**: ~103 occurrences updated across VM hot path

**Impact**: ⭐⭐⭐⭐ HIGH - Most frequently accessed variables in Lua VM
- Currently: `cl`, `k`, `base`, `pc`, `trap` (require domain knowledge)
- After: Self-documenting names (crystal clear semantics)

**Files Changed**: src/vm/lvirtualmachine.cpp (1 file, ~103 uses)
- Variable declarations (lines 97-101)
- Lambda definitions (register access, state management, control flow)
- Variable initialization (startfunc/returning labels)
- Main execution loop (~1054 lines)

**Testing**: All tests pass ✅ (2.22s)
**Benchmark**: 10-run average = 2.18s ✅ (only 3.3% slower than 2.11s, well within tolerance!)
**Performance**: Still 48% faster than 4.20s baseline! Target was ≤4.33s, we're at 2.18s.
**Risk**: HIGH (VM hot path, 100+ uses) - but performance maintained!

This was the highest-risk identifier modernization phase (VM core variables),
but the performance impact was minimal (~3.3% regression) and still dramatically
faster than baseline. The clarity improvement is substantial.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Peter Neiss