feat: Streaming JSON Parsing API

[vnixx]

Summary

Add streaming/incremental JSON parsing support to ReerJSON.

New Types

Bottom layer (JSONValue):

• JSONStreamParser — push-based streaming parser with two modes:
  • .jsonLines: extract multiple JSON documents from a byte stream (NDJSON/SSE)
  • .jsonArray: parse elements from a large JSON array one by one, O(1) memory
• JSONIncrementalReader — accumulate chunks for large single-document parsing

Codable layer:

• StreamingJSONLinesDecoder<T> / StreamingJSONArrayDecoder<T> — typed streaming decoders

AsyncSequence adapters:

• JSONValueStream / JSONValueByteStream / DecodingStream
• AsyncSequence.jsonValues() / .decode() convenience extensions

Implementation

• Uses yyjson's YYJSON_READ_STOP_WHEN_DONE + yyjson_doc_get_read_size() for accurate byte-level buffer management
• Internal buffer with lazy compaction (readOffset > buffer/2 triggers memmove)
• JSON Array mode uses a state machine to strip [, ,, ] and parse each element individually
• Zero impact on existing parsing paths

Testing

• 33 new tests covering JSON Lines, JSON Array, incremental reader, edge cases, and Codable layer
• All 755 tests pass (722 existing + 33 new), zero regressions

Note on yyjson_incr_* API

yyjson 0.12.0's incremental API (yyjson_incr_new/read/free) requires all data to be pre-loaded in the buffer — len only controls how far each parse step reads. It cannot handle dynamically appended data between reads. Therefore, network streaming uses STOP_WHEN_DONE instead.