Validation Generator: enforce model metadata attributes and documentation

[ooples]

Context

Child issue of #956 (Epic: Model Metadata Attributes & Source Generator Infrastructure)

Depends on: #957 (attributes must exist first)

Task

Create a Roslyn incremental source generator in AiDotNet.Generators that validates all model classes have proper metadata attributes and documentation at compile time.

Validations to Implement

Required Attributes (Compile Error)

For every non-abstract class that inherits from a model base type or implements a model interface:

• [ModelDomain] - at least one domain must be specified
• [ModelCategory] - exactly one category
• [ModelTask] - at least one task
• [ModelComplexity] - exactly one complexity level
• [ModelInput] - exactly one input/output type declaration

Emit diagnostic: AIDN001: Model class '{ClassName}' is missing required attribute '[{AttributeName}]'

XML Documentation (Compile Warning)

• Missing <summary> tag: AIDN010: Model class '{ClassName}' is missing XML doc summary
• Missing <remarks> with "For Beginners" content: AIDN011: Model class '{ClassName}' is missing beginner-friendly remarks
• Missing <example> block: AIDN012: Model class '{ClassName}' is missing usage example
• Public methods without <summary>: AIDN013: Public method '{MethodName}' on '{ClassName}' is missing XML doc
• Constructor parameters without <param>: AIDN014: Parameter '{ParamName}' on '{ClassName}' constructor is missing doc

Semantic Validation (Compile Warning)

• [ModelPaper] URL validation: AIDN020: ModelPaper URL '{Url}' is not well-formed (must start with https://)
• Domain/Task consistency: AIDN021: Model '{ClassName}' has domain '{Domain}' but no related task

Model Base Types to Scan For

The generator should detect classes that inherit from or implement:

• NeuralNetworkBase<T>
• IFullModel<T, TInput, TOutput>
• IModel<TInput, TOutput, TMetadata>
• IClassifier<T>
• ISyntheticTabularGenerator<T>
• AgentBase<T>
• SyntheticTabularGeneratorBase<T>
• CausalModelBase<T>
• GaussianProcessBase<T>
• Other model base types discovered during implementation

Technical Requirements

• Use IIncrementalGenerator (not ISourceGenerator) for performance
• Use syntax-first filtering (check for class declarations with base types) before semantic analysis
• Diagnostics should have proper severity (Error for missing attributes, Warning for missing docs)
• Must work with both net10.0 and net471 targets
• Should not significantly increase build time (< 2 seconds added)

Acceptance Criteria

• Generator discovers all model classes via base type/interface detection
• Missing required attributes produce compile errors
• Missing documentation produces compile warnings
• Paper URL validation works
• Domain/Task consistency check works
• Build time impact is < 2 seconds
• Generator itself has unit tests