feat: add serialization configuration for sorting map keys

[liuq19]

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Note

Adds a configurable option to sort map keys during JSON serialization, with supporting formatter changes, tests, and a minor tooling script; bumps version to 0.5.6.

• Serialization (serde):
  • Configurable sorting: Introduce SerializeCfg and Serializer::sort_map_keys() to sort map keys during serialization.
  • Implementation: Add MapState::{Stream, Sorted} and write_sorted_entries to buffer, sort, and emit entries; require Formatter: Clone to serialize buffered values.
  • Pretty output: Use PrettyFormatter::new() in to_writer_pretty.
  • Tests: Add tests validating default vs sorted behavior and feature sort_keys interaction.
• Tooling:
  • Add scripts/fmt_fix.sh for formatting (with --check support).
• Version:
  • Bump crate to 0.5.6.

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[PureWhiteWu]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

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[cxzl25]

Can we create a git tag?