Implement native SSE handling with configurable stream modes (once/continuous), automatic reconnection with exponential backoff, and lifecycle events.
Streams are configured via hx-stream attribute or htmx.config.streams global defaults.
Update test infrastructure with ReadableStream-based mocking utilities.
# Conflicts:
# src/htmx.js
- replace previous implementation with a simple fetch-based approach
- long-lived streams still to be implemented using @microsoft/fetch-event-source, but SSE tests need to be restructured
