Add validation rules, lazy properties, and readonly properties

[dereuromark]

Adds three new features to close competitive gaps with libraries like spatie/laravel-data and cuyz/valinor:

Validation Rules

Fields now support built-in validation constraints via the config builder:

Field::string('name')->required()->minLength(2)->maxLength(100)
Field::string('email')->pattern('/^[^@]+@[^@]+$/')
Field::int('age')->min(0)->max(150)
Field::float('score')->min(0.0)->max(100.0)

Rules are checked during construction (both fast-path and runtime). Null fields skip validation. On failure, throws InvalidArgumentException with descriptive message.

Lazy Properties

DTO and collection fields can defer hydration until first getter access:

Field::dto('customer', 'Customer')->asLazy()
Field::collection('items', 'OrderItem')->singular('item')->asLazy()

Raw array data is stored in $_lazyData during construction. The getter hydrates on demand. toArray() returns raw data directly if not yet hydrated — avoiding unnecessary object creation for pass-through scenarios.

Readonly Properties

DTOs can generate public readonly properties for language-level immutability:

Dto::create('Config')->readonlyProperties()->fields(
    Field::string('host')->required(),
    Field::int('port')->default(8080),
)

Generates public readonly ?string $host instead of protected ?string $host. The with*() methods use reconstruct-from-array to work around PHP's readonly limitations. Implies immutable.

Changes

• FieldBuilder: validation rule methods (minLength, maxLength, min, max, pattern) and asLazy()
• DtoBuilder: readonlyProperties() method (implies immutable)
• Builder/FieldCompletor: new field defaults and metadata keys
• Dto.php: validation rules in validate(), $_lazyData for lazy fields
• Templates: readonly property declarations, lazy getters, reconstruct-from-array with*() methods
• Tests: 38 new tests (ValidationTest, LazyTest, ReadonlyTest, config builder tests)
• Docs: updated ConfigBuilder.md and Validation.md

[codecov]

Codecov Report

❌ Patch coverage is 95.89041% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.93%. Comparing base (4496cdb) to head (48f9927).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Engine/XmlEngine.php	60.00%	2 Missing ⚠️
src/Generator/Builder.php	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #78      +/-   ##
============================================
+ Coverage     82.56%   82.93%   +0.37%     
- Complexity     1333     1375      +42     
============================================
  Files            40       40              
  Lines          3252     3323      +71     
============================================
+ Hits           2685     2756      +71     
  Misses          567      567              

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