perf: rm RawSourceMap

[SyMind]

This PR removes the RawSourceMap intermediary to parse source maps directly from simd_json BorrowedValue for performance, and updates constructors and serialization accordingly.

• Replace RawSourceMap with direct parsing from &mut [u8] using simd_json::to_borrowed_value
• Update SourceMap::from_json/from_slice/from_reader signatures and implementations
• Switch serialization call (but introduced a potential misuse of simd_json::to_string)