docs(symfony): add framework playground

[sidux]

Summary

• add a standalone Symfony editor playground covering PHP, YAML, XML, Twig, translations, Doctrine, forms, validation, events, Messenger, and local configuration schemas
• include a locked Composer setup while keeping runtime dependencies optional for basic editor exploration
• document useful files and navigation scenarios
• keep the example syntax-clean on PHP 8.2

Why

Cross-language framework intelligence is difficult to evaluate from isolated unit fixtures alone. A compact standalone workspace gives users a concrete place to try each interaction and gives maintainers one coherent project for manual regression checks without requiring a bootable application, database, or kernel.

Dependencies

• Depends on feat(symfony): add forms, validation, and config schema intelligence #404 so every feature demonstrated by the playground exists in the preceding Symfony stack.

Validation

• PHP 8.2 syntax lint for all 11 PHP example files
• cargo clippy --fix --allow-dirty -- -D warnings
• cargo fmt
• cargo clippy --all-targets -- -D warnings
• cargo test — 14,170 passed, 12 ignored

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.90452% with 710 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/code_lens.rs	88.14%	131 Missing ⚠️
src/completion/symfony.rs	85.45%	105 Missing ⚠️
src/symfony/container.rs	74.69%	104 Missing ⚠️
src/rename/prepare.rs	55.78%	65 Missing ⚠️
src/indexing/watch.rs	47.78%	59 Missing ⚠️
src/symfony/events.rs	93.90%	50 Missing ⚠️
src/symfony/expressions.rs	93.00%	45 Missing ⚠️
src/references/dispatch.rs	66.29%	30 Missing ⚠️
src/proxy_metadata.rs	92.13%	25 Missing ⚠️
src/references/members.rs	94.19%	25 Missing ⚠️
... and 10 more

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