feat: add SSE streaming helper support

Add Server-Sent Events (SSE) and streaming response support with three
streaming functions:

- stream(): Basic binary streaming with chunked transfer encoding
- stream_text(): Text streaming with proper Content-Type headers
- stream_sse(): SSE event streaming with full event field support

Features:
- SSEEvent struct with data, event, id, retry fields
- StreamContext for write, writeln, write_sse, sleep, pipe operations
- Multi-line data handling in SSE events
- Custom error handler support
- Client disconnect detection via on_abort callback
- Integrated with both Picoev and uSockets backends

New files:
- streaming.v: Core streaming implementation
- examples/sse_demo.v: Comprehensive SSE demo with test page
- tests/test_stream_*.v: 62 property-based tests

Updated:
- picoev_server.v: PicoevStreamWriter integration
- usockets_server.v: UsocketsStreamWriter integration
- README.md: SSE streaming documentation

.gitignore

@@ -46,4 +46,5 @@ temp/
 # Chunk upload temp files
 chunks/
 
-.kiro
+.kiro
+*.def