feat(serializer)!: reconcile nested translation writes in place

[paullla]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	yes
Deprecations?	no
Issues	fixes #61, relates to #36, #54
License	MIT

Problem

API Platform denormalizes the locale-keyed translations map as plain objects with no identity. On a write it recreates every translation row (ignoring the submitted id) and orphan-removes any locale missing from the payload, so a PATCH meant to edit one locale silently drops the rest, and re-writing a resource violates the (translatable_id, locale) unique constraint (#61) and churns translation ids (#36, #54).

Changes

• Add TranslatableItemDenormalizer: it intercepts a translatable payload carrying translations, lets API Platform build and populate the entity, then reconciles the map itself: populate the existing row per locale in place (stable id), create only genuinely new locales, keep locales absent from a PATCH, and remove them on PUT (HTTP replace semantics).
• Match existing translations by iterating and comparing getLocale() rather than getTranslations()->get($locale), so behavior does not depend on the user mapping indexBy: locale. A plain OneToMany hydrates as a 0-indexed list, and the model trait already matches the same way.
• Treat the map key as the authoritative locale: a locale in the request body no longer relabels (and, on PUT, orphan-removes) the row addressed by the key.
• PUT requires standard_put to be disabled, per operation (extraProperties: ['standard_put' => false]) or globally (api_platform.defaults.extra_properties.standard_put: false): with standard_put on, API Platform deserializes into a fresh object and its Doctrine PersistProcessor reflection-copies every property, including the translations collection, over the managed entity, so rows cannot be matched at any layer. The denormalizer detects that misconfiguration and throws a LogicException naming the fix, instead of letting the write die in the persist layer with an obscure Doctrine error. PATCH (the recommended edit verb) needs no configuration. Documented in the README and UPGRADE-2.0.
• Tests: unit tests for the merge, replace, create, in-place-id, list-shaped and misconfigured-PUT paths, plus a Doctrine integration test (in-memory sqlite and the real Symfony serializer) asserting id stability, orphan removal and map-key precedence against hydrated PersistentCollections, with the fixture intentionally not indexed by locale.
• CI: raise the PHPStan memory limit, needed now that the first mapped entities give phpstan-doctrine real work, and add one scoped ignore for the interface-typed owning side that property invariance forbids narrowing in the fixture.

Verification

• bin/phpunit: 65 tests, 129 assertions, green (9 pre-existing deprecations from legacy data providers).
• bin/phpstan analyse --memory-limit=512M: no errors (level 6).
• bin/php-cs-fixer check: no violations.
• composer validate --strict: valid.
• Rebased on upstream/master (e071ef7); CI green on the full matrix including the lowest-dependencies leg.