facet-rs / facet / 15436117135

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Fix ASCII control character hex escaping bug and add comprehensive tests

- Fixed critical bug in write_json_escaped_char where raw bytes were used
  instead of extracting hex digits (nibbles) from Unicode code points
- The bug caused panic when serializing control characters like \u{7F}
  because char::from_digit(127, 16) returns None (127 > 15)
- Now correctly extracts 4-bit nibbles using bit shifts and masking
- Added comprehensive test suite covering:
  * All ASCII control characters (0x00-0x1F, 0x7F)
  * Special escape sequences remain unaffected (\n, \r, \t, etc.)
  * Mixed strings with control characters
  * Roundtrip serialization/deserialization
  * Hex digit generation edge cases
- All existing facet-json tests continue to pass

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