feat: cache the parsed translation files, keyed by their content

[JDCyllene]

What this pull request does

Lot C of the changes the Sylius integration asked for: the bundle no longer parses every translation file again in each new process.

1. Parsed files are cached in the pool cyllene_digital_ai_translation.translation_files_cache, under a key made of the file's path, a hash of its content and a format version (TranslationFileScanner::CACHE_FORMAT). Nothing invalidates an entry: a file whose content changes gets a new key, and the pool's 7-day lifetime removes what is left behind. The walk of the directories is not cached, so a file added or removed is seen at once. A file that cannot be parsed is cached as such, and still reported in the ignored files when served from the cache. A file that cannot be read is reported by the scanner itself and never cached.
2. The pool is declared by the bundle with adapter: cache.app and a 7-day lifetime (prepended into framework.cache.pools); a host entry with the same name changes either. Its name joins the public contract.
3. translation_files_cache (boolean, default true): false parses the files in every new process, as before.
4. TranslationFileScanner implements ResetInterface: its in-memory memo is emptied between two Messenger messages, so a worker sees an edited file without a restart.

Measured on the 58 XLIFF files of symfony/demo (3,191 entries): about 230 ms to scan without the cache, about 5 ms with it (content hashed).

ADR 0027; docs/operations/cache.md, docs/configuration-reference.md, docs/operations/multi-server.md, docs/public-contract.md and CLAUDE.md are updated.

Points to watch

• Tests pass (vendor/bin/phpunit): SQLite, MariaDB 11.8, PostgreSQL 17, without intl and with the lowest dependencies
• PHPStan and PHP-CS-Fixer are green (vendor/bin/phpstan analyse, vendor/bin/php-cs-fixer check)
• UPGRADE.md is up to date if the public contract changes: an addition only, nothing to upgrade
• A decision taken or reversed has its record in docs/adr/: 0027
• No real API key appears in the code, the tests or this description

No change to the entity mapping. The configuration tree gains one optional key with a default: the Flex recipe stays as it is.

For review:

• After review, the key is the content's hash rather than the date of modification and the size: a file rewritten within the same second at the same size, or a release deployed under the same path with the dates a build froze on a shared pool, is no longer served stale. ADR 0027 lists the date and size among the alternatives rejected.
• The loaders' version is not part of the key: a parsing change in symfony/translation is seen once the entry expires, or with bin/console cache:pool:clear cyllene_digital_ai_translation.translation_files_cache.
• On Symfony 7.4+, APP_SHARE_DIR (set by the skeleton to var/share) holds cache.app, which then survives cache:clear: docs/operations/cache.md says to empty the pool with the command above.
• Tested in a copy of the sandbox (prod): an edited XLIFF file is seen at once, cache:pool:clear empties the pool, and a host entry under framework.cache.pools overrides the adapter while the lifetime is kept.