Integrate Respect/Fluent and Respect/FluentGen

Replace the in-house factory and codegen infrastructure with Fluent and
FluentGen packages. The runtime now uses ComposingLookup with ComposableMap
for prefix resolution via a FluentValidatorFactory adapter that preserves
exception types. Validators use #[Composable] from Fluent instead of the
custom #[Mixin] attribute. Mixin generation uses FluentGen, producing
PrefixConstants in place of PrefixMap. The old src-dev/CodeGen directory is
removed. All public APIs and BC are preserved.

Author: alganet

composer.json

@@ -23,6 +23,7 @@
         "php": ">=8.5",
         "php-di/php-di": "^7.1",
         "psr/container": "^2.0",
+        "respect/fluent": "^1.0",
         "respect/string-formatter": "^1.7",
         "respect/stringifier": "^3.0",
         "symfony/polyfill-intl-idn": "^1.33",
@@ -48,6 +49,7 @@
         "sokil/php-isocodes-db-only": "^4.0",
         "squizlabs/php_codesniffer": "^4.0",
         "symfony/console": "^7.4 || ^8.0",
+        "respect/fluentgen": "^1.0",
         "symfony/translation": "^8.0",
         "symfony/var-exporter": "^6.4 || ^7.0 || ^8.0"
     },