Add PHPStan level 8 + PHP-CS-Fixer; fix surfaced issues (roadmap §7)

[mmucklo]

Addresses ROADMAP §7 (first pass — phpstan + cs-fixer).

Tooling added

• phpstan/phpstan ^2.1 as dev dep. Config at `phpstan.neon.dist`: level 8, scans `src/` and `tests/`.
• friendsofphp/php-cs-fixer ^3.95 as dev dep. Config at `.php-cs-fixer.dist.php`:
  • `@PSR12`
  • `declare_strict_types`
  • `no_unused_imports`
  • `ordered_imports`
  • `single_quote`
  • `trailing_comma_in_multiline`
• Both wired into `.github/workflows/ci.yml` on the PHP 8.3 matrix leg. phpstan uses `--error-format=github` for inline PR annotations. cs-fixer runs `--dry-run --diff`.
• `.gitignore`: `.php-cs-fixer.cache`.

Issues phpstan surfaced (and fixes)

1. Missing value types on array properties/returns (8 errors)

Added `@var array<K, V>` on all static rule tables, caches, and data-provider return types.

2. `preg_replace` can return null (5 errors)

PHP's `preg_replace` returns `string|null` — `null` on regex engine error. We control all patterns so null is effectively impossible in practice, but the type system correctly flags it. Fall back to the input string via `?? $string` at all three call sites:

```php

• self::$pluralCache[$string] = preg_replace($pattern, $result, $string);

• self::$pluralCache[$string] = preg_replace($pattern, $result, $string) ?? $string;
  ```

Issues cs-fixer surfaced (and fixes)

• Double-quoted literals with regex backreferences (`"$1zes"`) converted to single-quoted (`'$1zes'`). PHP doesn't interpolate `$1` since a digit can't start an identifier, so behavior is identical; single-quoted is idiomatic for non-interpolated strings.
• One whitespace fix around a fat arrow.

Test plan

• 117 tests / 118 assertions pass on PHP 8.1 and 8.3.
• phpstan level 8: No errors.
• cs-fixer dry-run: clean.
• CI green on PHP 8.1–8.4 + new phpstan/cs-fixer steps on 8.3.

Deferred

Roadmap §7 also calls for `infection` (mutation testing) and `phpbench` (benchmarks). Keeping those out of this PR to avoid ballooning scope — can land as separate PRs.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.07%. Comparing base (a5bba46) to head (43b5fe2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #18   +/-   ##
=========================================
  Coverage     98.07%   98.07%           
  Complexity       27       27           
=========================================
  Files             1        1           
  Lines            52       52           
=========================================
  Hits             51       51           
  Misses            1        1           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.