Fix singleton unwrapping for $each, $spread, and $match

[NirBarak-RecoLabs]

Summary

• $each, $spread, and $match returned []any instead of *Sequence, which prevented CollapseSequence from performing singleton unwrapping. Per the JSONata spec, when a function returns a single-element sequence the result must be unwrapped to its scalar value.
• The KeepArray operator ([] suffix) now correctly sets KeepSingleton on the Sequence before CollapseSequence runs, ensuring single-element arrays are preserved when the operator is applied.

What changed

File	Change
functions/object_funcs.go	$each and $spread return *Sequence instead of []any
functions/string_match_replace.go	$match returns *Sequence instead of []any
internal/evaluator/eval_function.go	Set KeepSingleton when KeepArray is true, before calling CollapseSequence
evaluator_test.go	New TestEachSingletonUnwrap covering scalar, array, nested, and error cases
testdata/groups/function-each/	5 new JSON suite tests (case003-case007)
testdata/groups/function-spread/	2 new JSON suite tests (case004-case005)
testdata/groups/function-match/	3 new JSON suite tests (case000-case002)

Examples

/* Before fix: returned ["a=1"], now correctly returns "a=1" */
$each({"a": 1}, function($v, $k) { $k & "=" & $string($v) })

/* Before fix: returned [{"a": 1}], now correctly returns {"a": 1} */
$spread({"a": 1})

/* Before fix: returned [{"match": "h", ...}], now correctly returns {"match": "h", ...} */
$match("hello", /^h/)

Test plan

• All existing JSON suite tests pass
• All existing Go unit tests pass
• New tests verify singleton unwrapping for $each, $spread, $match
• New tests verify KeepArray operator preserves arrays correctly
• Verified parity with jsonata-js reference implementation