Register missing fixers in default set, enforce CS in CI, fix README

[TomasVotruba]

What & why

Three fixes found while reviewing the repo:

1. Register two documented-but-unshipped fixers

StandaloneLineInMultilineArrayFixer and StandaloneLineConstructorParamFixer are documented in the README and fully tested, but were missing from config/symplify.php — so they never ran for users of SetList::SYMPLIFY. Now registered.

2. CI was not enforcing coding standard

The Coding Standard job ran composer fix-cs (ecs --fix), which auto-corrects and exits 0 — style regressions could never fail the build. Switched to composer check-cs. For a coding-standard package this is the most important check to actually enforce.

3. CI hardening

• Added a tests job with a PHP matrix (8.2, 8.3, 8.4) — previously tests only ran on 8.2, despite >=8.2 support and version-conditional code paths.
• Bumped actions/checkout@v3 → v4 and ramsey/composer-install@v2 → v3.

4. README accuracy

• Header count 12 Rules → 14 Rules (all 14 fixers are documented).
• Fixed a broken class link: RemoveRedundantDescriptionFixer (does not exist) → RemoveMethodNameDuplicateDescriptionFixer.

Verification

Local run all green: PHPStan (lvl 8), ECS, Rector, PHPUnit (150 tests).

[TomasVotruba]

LGTM 👍