docs: add PHPStan 2.x migration guide and baseline reduction strategy

[CybotTM]

Summary

Adds two new sections to the PHPStan compliance reference:

• Baseline Reduction Strategy — patterns for genuinely fixing PHPStan errors instead of using @phpstan-ignore suppressions (is_string/is_array guards, step-by-step JSON/YAML validation, return type covariance, type-specific parameters)
• PHPStan 1.x to 2.x Migration — breaking changes table (config renames, new error identifiers, symplify/phpstan-rules 14.x dropped rules) and step-by-step migration checklist

Based on real-world experience upgrading render-guides (TYPO3-Documentation/render-guides) from PHPStan 1.x to 2.x, reducing baseline from 41 to 9 entries with zero @phpstan-ignore annotations.

Test plan

• Content is valid markdown
• Code examples are syntactically correct
• Renders correctly in skill documentation

[gemini-code-assist]

Summary of Changes

Hello @CybotTM, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the PHPStan compliance reference documentation by adding two crucial sections. The first provides a strategic approach to reducing PHPStan baselines through genuine code fixes, aiming to minimize the reliance on @phpstan-ignore annotations. The second offers a detailed migration guide for upgrading projects from PHPStan 1.x to 2.x, outlining breaking changes and a clear checklist to facilitate a smooth transition. These additions aim to improve code quality and streamline the upgrade process for PHP projects.

Highlights

• Baseline Reduction Strategy: Added a new section detailing patterns for genuinely fixing PHPStan errors, emphasizing code fixes over suppressions. This includes strategies for handling mixed types using is_string()/is_array() guards, step-by-step validation for json_decode()/Yaml::parse(), return type narrowing (covariance), and using type-specific parameters.
• PHPStan 1.x to 2.x Migration Guide: Introduced a comprehensive guide for migrating from PHPStan 1.x to 2.x. This covers breaking changes such as config renames (strictCalls to strictFunctionCalls), stricter mixed type handling, new error identifiers, and dropped rules from symplify/phpstan-rules. It also provides a step-by-step migration checklist.

Changelog

• skills/php-modernization/references/phpstan-compliance.md
  • Added a new section 'Baseline Reduction Strategy' with code examples.
  • Added a new section 'PHPStan 1.x to 2.x Migration' including breaking changes and migration steps.

Activity

• No human activity has occurred on this pull request yet.

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[gemini-code-assist]

Code Review

The pull request provides a clear and helpful guide for migrating to PHPStan 2.x and managing the baseline. The strategies for fixing mixed types are well-explained and align with modern PHP development practices. I have suggested a small refinement to the JSON validation example to replace a PHPDoc type hint with a runtime guard, which is more robust and consistent with the 'genuine fix' philosophy described in the document.