SSE parser byte-level optimization, chunked decoding, auto-reconnect

[Copilot]

Implements the four iOS-specific optimizations discussed in the issue: reducing ARC/CPU overhead in the SSE parser, handling proxied chunked streams, seamless reconnection on network switches, and thread-safety documentation.

SSEParser: byte-level accumulation

• lineBuffer changed from String to Data; line-ending scan uses raw bytes (0x0A/0x0D)
• Field names compared as [UInt8] arrays — no String allocation per line
• String(data:encoding:.utf8) deferred to event dispatch boundary (\n\n)

ChunkedDecoder (new)

• Incremental state-machine that strips Transfer-Encoding: chunked framing before SSE parsing
• Handles partial chunks split across TCP segments, chunk extensions, hex size variants
• Integrated via enableChunkedDecoding() on NWAsyncSocket

SSE auto-reconnect with Last-Event-ID

• enableSSEAutoReconnect(retryInterval:) — on error disconnect, schedules reconnect preserving lastEventId
• Honors server-sent retry: field for backoff interval
• Delegate callback socket(_:willAutoReconnectWithLastEventId:afterDelay:) lets the app re-send the HTTP request with Last-Event-ID header
• Explicit disconnect() cancels auto-reconnect

Thread safety docs

• Documented socketQueue / delegateQueue separation in class-level and method-level doc comments (behavior was already correct)

let socket = NWAsyncSocket(delegate: self, delegateQueue: .main)
socket.enableSSEParsing()
socket.enableChunkedDecoding()        // behind Nginx/CDN
socket.enableSSEAutoReconnect()       // auto-retry with Last-Event-ID
try socket.connect(toHost: "api.example.com", onPort: 443)

87 tests pass (71 existing + 16 new ChunkedDecoder tests). All existing SSEParser/StreamBuffer/ReadRequest tests unchanged.

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