--json: preserve XML element order

[bukzor]

Summary

XML element order is now preserved when converting to JSON. Previously, keys were alphabetically sorted due to Go's map iteration order.

Problem

XML element order is semantically significant. For example:

• CSV-like data where column order matters
• Serialized objects where field order has meaning
• Configuration files where order affects behavior

Previously, xq's JSON output lost this ordering information:

echo '<root><name>a</name><msg>b</msg><args>c</args></root>' | xq -j
# Before: {"root":{"args":"c","msg":"b","name":"a"}}  # alphabetical

Solution

Added OrderedMap type that preserves insertion order by implementing custom json.Marshaler:

echo '<root><name>a</name><msg>b</msg><args>c</args></root>' | xq -j
# After: {"root":{"name":"a","msg":"b","args":"c"}}  # document order

Changes

• Added OrderedMap type with custom MarshalJSON() implementation
• Updated NodeToJSON() to return *OrderedMap instead of map[string]interface{}
• Updated nodeToJSONInternal() to use OrderedMap
• Updated addToResult() to work with *OrderedMap
• Added TestElementOrderInJSON to verify ordering is preserved
• Updated TestCDATASupport to work with OrderedMap
• Updated formatted3.json test fixture to match document order

Breaking Change

This is technically a breaking change for code that:

• Uses type assertions to check for map[string]interface{}
• Relies on alphabetical ordering of keys

However, the previous behavior was arguably a bug, and most code should work with the new OrderedMap type since it marshals to standard JSON.

🤖 Generated with Claude Code

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.95238% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.83%. Comparing base (ec5a59a) to head (bc62ef3).

Files with missing lines	Patch %	Lines
internal/utils/jsonutil.go	80.95%	5 Missing and 3 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   80.57%   80.83%   +0.25%     
==========================================
  Files           5        5              
  Lines         690      720      +30     
==========================================
+ Hits          556      582      +26     
- Misses         92       94       +2     
- Partials       42       44       +2     

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