Fix json_encode enum repro/compliance — UnitEnum name clash (#6130)

[PurHur]

Summary

• Closes Stdlib: json_encode() — enum case operands must match Zend JSON output (ext/json/php_json.c) #6130
• VmJson::export() / StringJsonEncode enum handling was already green on master; repro and VM compliance PHPT failed because enum UnitEnum clashes with PHP's built-in UnitEnum interface (exit 255 before json_encode ran).
• Renamed unit enum to UE in repro + compliance; added JIT compliance PHPT.

PHP implementation (already on master)

• ext/standard/VmJson.php::exportEnumCase() — unit "", backed scalar
• lib/JIT/Builtin/StringJsonEncode.php — TYPE_ENUM_CASE branch

C runtime shrink

No new C; lib/AOT/runtime/ remains phpc_progress.c only.

Verification

docker info >/dev/null
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/parity_json_encode_enum.php
php bin/vm.php test/repro-maintainer/parity_json_encode_enum.php 2>/dev/null | tail -3
vendor/bin/phpunit --filter json_encode_enum
'

Output:

unit: 
backed_string: "x"
backed_int: 1
unit: ""
backed_string: "x"
backed_int: 1
OK (2 tests, 2 assertions)

Test plan

• Maintainer repro Zend + VM side-by-side
• json_encode_enum VM + JIT compliance filter

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