Introduce coding-standards tooling (php-cs-fixer, PER-CS) with CI enforcement

[lisachenko]

Motivation

The repo has no CS tooling at all — no .php-cs-fixer.dist.php, .phpcs.xml, rector.php, or .editorconfig — and drift is already visible: bin/aspect:4 uses declare(strict_types = 1) (spaces) while src/ consistently uses declare(strict_types=1). Notably, CI already installs cs2pr (.github/workflows/phpunit.yml:53) but nothing ever uses it.

Proposed change

• Add friendsofphp/php-cs-fixer (dev dep) with a @PER-CS ruleset + declare_strict_types and risky rules kept minimal.
• Add .editorconfig matching current conventions (4 spaces, LF, final newline).
• Add a CI cs job running php-cs-fixer check --diff reported through cs2pr.
• One-off php-cs-fixer fix commit, kept separate from the config commit for reviewability.

Acceptance criteria

• composer cs / composer cs:fix (or documented equivalent) work locally; CI fails on style violations.
• The fix commit produces no behavioral diff (phpunit + PHPStan level 10 green).

Note: should land after the other open modernization PRs to avoid repo-wide formatting conflicts.