Add interactive demo apps for Swift and Objective-C

- Swift demo: Examples/SwiftDemo/main.swift (SPM executable target)
- ObjC demo: ObjC/ObjCDemo/main.m (build with clang on macOS)
- Both demos cover: StreamBuffer, SSEParser, UTF-8 safety, ReadRequest, socket usage
- Interactive menu-driven interface for testing individual components
- Updated README with demo instructions and file structure

Co-authored-by: dustturtle <2305214+dustturtle@users.noreply.github.com>

Author: Copilot