feat(benchmark): add comprehensive performance benchmarking system

🚀 Phase 8.2 PoC2 Achievement: 280x WASM performance boost proven\!

## New Features:
- Complete benchmark framework (src/benchmarks.rs)
- CLI integration: --benchmark --iterations options
- 3-backend comparison: Interpreter/VM/WASM
- Automated performance measurement & reporting

## Benchmark Results (100 iterations average):
- WASM: 0.17ms (280x faster than Interpreter\!)
- VM: 16.97ms (2.9x faster than Interpreter)
- Interpreter: 48.59ms (baseline)

## Added Files:
- benchmarks/bench_{light,medium,heavy}.nyash - Test cases
- benchmark_summary_20250814.md - Clean results
- wasm_demo/ - Browser execution environment

## Documentation Updates:
- docs/execution-backends.md - Added benchmark usage
- docs/CURRENT_TASK.md - Phase 8.3 Copilot coordination
- CLAUDE.md - Quick benchmark access

## Copilot Integration Ready:
- Phase 8.3 merge conflict avoidance strategy documented
- Benchmark framework ready for Box operation performance validation
- CLI integration preserved for future enhancements

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

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

benchmarks/bench_medium.nyash

@@ -0,0 +1,51 @@
+// Medium benchmark - more calculations
+static box Main {
+    main() {
+        local sum, temp, i
+        sum = 0
+        i = 1
+        
+        // Manual "loop" - 10 iterations of calculations
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        i = i + 1
+        
+        temp = i * 2 + 3
+        sum = sum + temp
+        
+        print(sum)
+        return sum
+    }
+}