feat: add one-to-many collection field support

[phmatray]

Summary

Adds collection field support to FormCraft, enabling one-to-many model forms (e.g., Order with OrderItems). Implements the fluent API proposed in #20.

Fixes #20

Changes

Core library (FormCraft/)

• Forms/Core/ICollectionFieldConfiguration.cs — New generic interface defining collection field config (accessor, setter, min/max items, add/remove/reorder flags)
• Forms/Core/ICollectionFieldConfigurationBase.cs — Non-generic base interface for type-erased storage in collections
• Forms/Core/CollectionFieldConfiguration.cs — Implementation with expression-based property access and setter compilation
• Forms/Core/IFormConfiguration.cs — Added ICollectionFormConfiguration<TModel> interface (non-breaking); FormConfiguration now implements it with CollectionFields list
• Forms/Builders/CollectionFieldBuilder.cs — Fluent builder with AllowAdd(), AllowRemove(), AllowReorder(), WithMinItems(), WithMaxItems(), WithItemForm()
• Forms/Builders/FormBuilder.cs — Added AddCollectionField<TItem>() method
• Forms/Validators/CollectionFieldValidator.cs — Validates min/max item counts and recursively validates each item's fields

MudBlazor layer (FormCraft.ForMudBlazor/)

• Features/CollectionField/CollectionFieldComponent.razor[.cs] — Renders collection with MudBlazor: add button, item cards with remove/reorder controls, per-item field rendering
• Features/CollectionField/CollectionFieldRenderer.cs — Reflection bridge from non-generic ICollectionFieldConfigurationBase to generic CollectionFieldComponent<TModel, TItem>
• Features/FormContainer/FormCraftComponent.razor — Renders collection fields after scalar fields
• Features/FormContainer/FormCraftComponent.razor.cs — Added CollectionConfiguration property and HandleCollectionChanged method
• Features/Validation/DynamicFormValidator.cs — Extended to validate collection fields recursively on form submission

Tests (FormCraft.UnitTests/)

• Builders/CollectionFieldBuilderTests.cs — 15 tests covering builder API, chaining, defaults, accessor, item form, and full API pattern matching owner's proposal
• Validators/CollectionFieldValidatorTests.cs — 5 tests covering min/max validation and per-item recursive validation

Testing

• 20 new unit tests added, all passing
• 557 existing tests remain passing (577 total, 0 failures)
• Build succeeds on all target frameworks (net8.0, net9.0, net10.0)
• Full API matches the owner's proposed pattern from issue comments

Discovered Issues

• The Title attribute on MudIconButton triggers MUD0002 warnings in the MudBlazor analyzer (pre-existing pattern in the codebase, not introduced by this PR)
• EditContext flat model design means nested field identifiers (e.g., Items[0].ProductName) are handled at the component level rather than through Blazor's built-in field identifier system — this works but could be improved in a future iteration
• Drag-and-drop reordering not included (uses up/down buttons as specified in the analysis)