Add synchronous SSEParser and lower deployment minimums

[zac]

Summary

• Add SSEParser as the synchronous incremental parser core for server-sent events
• Rebuild async parsing on top of the parser and expose .sse() on any AsyncSequence<UInt8>
• Keep reconnect state available directly from the parser and update EventSource to read it synchronously
• Lower platform minimums in Package.swift and add the shared Xcode scheme and license files
• Document the new parser workflow in the README and plan the 0.2.0 parser split

Testing

• Added parser coverage for synchronous vs async parity, chunk-boundary invariance, state-only blocks, finish/reuse behavior, and trailing retry handling
• Updated line-splitting tests to exercise the new synchronous splitter directly
• Not run (not requested)