Declare unused private promoted properties as readonly (PHP 8.1+)

[lacatoire]

Summary

This PR marks all private promoted properties that are never reassigned as readonly.

Motivation

• Improves immutability guarantees for internal classes.
• Helps static analyzers (PHPStan, Psalm) infer data flow more accurately.
• Aligns PhD with modern PHP conventions and safety features introduced in PHP 8.1.

Scope

• Scanned all constructor-promoted properties.
• Identified private ones never written to outside the constructor.
• Declared them readonly.

Example

Before:
class Reader {
public function __construct(private string $path) {}
}

After:
class Reader {
public function __construct(private readonly string $path) {}
}

Impact

• ✅ No behavior change.
• ✅ Pure internal safety improvement.
• ✅ Forward-compatible with PHP 8.4+.
• 🧠 Enables better static analysis and type inference.

References

• PHP 8.1: readonly properties RFC