Locale support: abstract Locale, En, instance API, extension methods (v2.1 — §5 + §5a)

[mmucklo]

Implements ROADMAP items §5 (extensibility) and §5a (locale-based inflections) per the design locked in #20.

New files

• `src/Locale/Locale.php` — abstract base class. Holds rule tables as `protected` instance arrays, inflection engine (`pluralize`/`singularize`), extension methods (`addIrregular`/`addUncountable`/`addPluralRule`/`addSingularRule`), per-instance caching with cache invalidation on mutation.
• `src/Locale/En.php` — English locale. Seeds rule tables from `protected const` class constants via constructor. Identical rule set to v2.0 (enriching from `mmucklo/inflections` deferred to a follow-up).

Changed files

• `src/Inflect.php` — rewritten as a facade. No breaking changes to the static API.

Static API (back-compat)

```php
Inflect::pluralize('cat'); // 'cats' — unchanged
Inflect::singularize('cats'); // 'cat' — unchanged
Inflect::pluralizeIf(3, 'person'); // '3 people' — unchanged
```
Internally delegates to a lazily-initialized shared `En` instance. New proxy extension methods:
```php
Inflect::addIrregular('dwarf', 'dwarves');
Inflect::addUncountable('moose');
Inflect::addPluralRule('/^(platypus)$/i', '$1es');
Inflect::addSingularRule('/(platypus)es$/i', '$1');
```

Instance API (new)

```php
$en = new Inflect(); // default 'en'
$en = new Inflect('en'); // explicit by name
$en = new Inflect(new En()); // explicit by object

$en->plural('cat'); // 'cats'
$en->singular('cats'); // 'cat'
$en->pluralIf(3, 'person'); // '3 people'
$en->getLocale()->addIrregular('formula', 'formulae');
```

Instances are isolated — extending one instance's rules doesn't affect another.

Locale registry

```php
Inflect::registerLocale('fr', Fr::class); // lazy: class instantiated on first use
$fr = new Inflect('fr');
```
Unknown locale throws `InvalidArgumentException`. Ships with `'en'` pre-registered.

Test plan

• All 117 existing tests still pass (back-compat verified).
• 12 new tests covering: instance API (plural, singular, pluralIf, construction from object, instance isolation), extension methods (addIrregular, addUncountable, addPluralRule, addSingularRule, cache invalidation), locale registration (register + resolve, unknown-locale exception), static extension proxy.
• 129 tests / 141 assertions pass on PHP 8.1 and 8.3.
• PHPStan level 8: no errors.
• PHP-CS-Fixer: clean.
• CI green on 8.1–8.4.

Follow-up (not in this PR)

• Enrich `En` rule set from `mmucklo/inflections` (richer irregulars/uncountables).
• Add at least one non-English locale (v2.2).
• Update README and CHANGELOG.md once merged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.45794% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.63%. Comparing base (82e3e89) to head (49a165e).

Files with missing lines	Patch %	Lines
src/Inflect.php	84.61%	6 Missing ⚠️
src/Locale/Locale.php	98.41%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #22      +/-   ##
============================================
- Coverage     98.07%   93.63%   -4.45%     
- Complexity       27       51      +24     
============================================
  Files             1        3       +2     
  Lines            52      110      +58     
============================================
+ Hits             51      103      +52     
- Misses            1        7       +6     

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