Stabilize classic menu cache keys and purge tracking

[mehul0810]

Summary

• replace full query-state menu cache keys with stable output-affecting signatures
• retain compatibility for query-aware walkers through an explicit filter and bounded custom segments
• cap tracked variants at 100, evict the oldest entry, recover safely from malformed tracking data, and purge by menu ID
• document the key contract and add unit plus disposable classic-theme browser coverage

Validation

• composer lint (91 PHP files)
• vendor/bin/phpunit (148 tests, 482 assertions)
• npm run test:js (11 suites, 40 tests)
• npm run lint
• npm run build
• php -d memory_limit=2G vendor/bin/phpstan analyse --no-progress --debug
• git diff --check

Closes #106

[github-actions]

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