Add execa-compatible API with superior streaming and virtual commands

[konard]

Goal: Beat execa's 98M weekly downloads by providing their full API plus unique features

Missing in execa:

• No virtual commands engine
• Limited streaming (no real-time processing)
• No built-in commands
• No mixed pipeline support (system + virtual)
• No async iteration over output
• Bundle size ~50KB+ vs our ~20KB

Implementation:

• Add $.execaCompat() method with full execa API
• Document feature-by-feature migration guide
• Add streaming benchmarks vs execa's buffering
• Show virtual commands in pipeline examples
• Create async iteration examples (unique to us)
• Bundle size comparison documentation

Competitive Advantages to Showcase:

• ✅ Real-time streaming + events (vs execa's limited streaming)
• ✅ Async iteration (for await (chunk of $.stream()))
• ✅ Virtual commands engine (revolutionary feature)
• ✅ Mixed pipelines (system + built-in + virtual)
• ✅ Built-in commands (18 vs 0)
• ✅ Smaller bundle (~20KB vs ~50KB+)

Success Metrics:

• Complete execa compatibility layer
• Migration guides for common patterns
• Performance benchmarks
• Unique feature demonstrations