Lexicon checker: cross-topic duplicate reports and element label cleanup

[Ibochkarev]

What changed and why

First slice of #14512. The core lexicon checker now reports keys that appear in more than one topic (identical value vs conflicting value). English element topics no longer repeat type labels (chunk/chunks, snippet/snippets, plugin/plugins, template/templates) that already exist in default, which the manager always loads.

CheckLexicon lives in MODX\Revolution\Build, with a thin CLI entrypoint and a short README for maintainers.

How to test

php -l _build/lexicon/checklexicon.class.php
php -l _build/lexicon/checklexicon.php
cd _build/test && ../../core/vendor/bin/phpunit --filter CheckLexiconDuplicateTest
# expect: OK (3 tests, 35 assertions), exit 0

php _build/lexicon/checklexicon.php en
# expect: exit 0; reports under _build/lexicon/_*.php (gitignored)

On a running manager install, confirm Elements tree labels still resolve when cultureKey=en (depends on core:default).

Related issue(s)/PR(s)

Refs #14512
Builds on the checker from #14979

Compatibility notes

• English core lexicons only in this PR (Crowdin for other languages).
• Keys remain available via default. Manager and connectors already load core:default.
• Loading an element topic alone without default no longer provides those eight type labels (documented trade-off).
• Does not mass-delete _superfluous keys (extras may still use them).

Breaking change assessment

No public PHP API change. Lexicon key availability for the eight labels still works when default is loaded. Topic-only loads without default lose those labels.

Test coverage

• _build/test/Tests/Build/CheckLexiconDuplicateTest.php (suite Build in phpunit.xml)
• Gate E this session: php -l exit 0; phpunit exit 0 (3/35)

Contributors

@Ruslan-Aleev @Jako @JoshuaLuckers

AI tool use

Cursor agent assisted implementation, tests, and PR drafting under maintainer review.