feat: YAML validation, NRE fix, Cookbook, llms updates

AGENTS.md

@@ -169,15 +169,15 @@ byte[] png = await render.Render(_templates["receipt"], data);
 | Configuration | `FlexRenderBuilder`, `SkiaBuilder`, `FlexRenderOptions`, `ResourceLimits` |
 | Abstractions | `IFlexRender`, `IResourceLoader` |
 | Parsing | `TemplateParser`, `Template`, `CanvasSettings`, `TextElement`, `FlexElement`, `QrElement`, `BarcodeElement`, `ImageElement`, `SeparatorElement`, `TableElement`, `TableColumn`, `TableRow`, `EachElement`, `IfElement`, `ContentElement` |
-| Template Engine | `TemplateExpander`, `TemplateProcessor`, `ExpressionLexer`, `ExpressionEvaluator`, `TemplateContext`, `InlineExpressionParser`, `InlineExpressionEvaluator`, `FilterRegistry`, `ITemplateFilter`, `ContentSourceResolver` |
+| Template Engine | `TemplateExpander`, `TemplateProcessor`, `ExpressionLexer`, `ExpressionEvaluator`, `TemplateContext`, `InlineExpressionParser`, `InlineExpressionEvaluator`, `FilterRegistry`, `ITemplateFilter` |
 | Layout | `LayoutEngine`, `LayoutNode`, `LayoutContext`, `LayoutSize`, `IntrinsicSize`, `Unit`, `UnitParser`, `MarginValue`, `MarginValues`, `PaddingParser.ParseMargin` |
 | Rendering (Skia) | `SkiaRender` (IFlexRender impl), `SkiaRenderer`, `TextRenderer`, `FontManager`, `ColorParser`, `RotationHelper`, `BmpEncoder`, `BoxShadowParser`, `GradientParser` |
 | Rendering (ImageSharp) | `ImageSharpRender` (IFlexRender impl), `ImageSharpRenderingEngine`, `ImageSharpTextRenderer`, `ImageSharpFontManager` |
 | Providers | `IContentProvider<T,O>`, `QrProvider`, `BarcodeProvider`, `ImageProvider` |
 | Loaders | `FileResourceLoader`, `Base64ResourceLoader`, `EmbeddedResourceLoader`, `HttpResourceLoader` |
 | DI | `ServiceCollectionExtensions.AddFlexRender()` |
 | Values | `TemplateValue` (abstract), `StringValue`, `NumberValue`, `BoolValue`, `NullValue`, `ArrayValue`, `ObjectValue` |
-| Content Parsers | `IContentParser`, `IBinaryContentParser`, `ContentParserRegistry`, `ContentSourceResolver`, `NdcContentParser` |
+| Content Parsers | `IContentParser`, `IBinaryContentParser`, `ContentParserRegistry`, `NdcContentParser` |
 
 ## Coding Conventions
 
@@ -409,9 +409,10 @@ var sb = new StringBuilder(estimatedCapacity);
 
 1. Create AST model in `Parsing/Ast/` (sealed class extending `TemplateElement`)
 2. Add parser function in `TemplateParser.cs` -- register in `_elementParsers` dictionary
-3. Add flex-item property support via `switch` pattern matching in layout engine
-4. Add rendering in `SkiaRenderer.RenderNode()` or create a provider
-5. Write tests for each step
+3. Register all YAML properties in `KnownProperties.cs` (for YAML validation and typo suggestions)
+4. Add flex-item property support via `switch` pattern matching in layout engine
+5. Add rendering in `SkiaRenderer.RenderNode()` or create a provider
+6. Write tests for each step
 
 ### Add new template expression
 
@@ -424,9 +425,10 @@ var sb = new StringBuilder(estimatedCapacity);
 
 1. Add property to `TemplateElement` in `Parsing/Ast/TemplateElement.cs`
 2. Parse the property in `ElementParsers.cs`
-3. Add the property to `TemplateElement.CopyBaseProperties()` (single source of truth in `Parsing/Ast/TemplateElement.cs`)
-4. If the property contains expressions (e.g., `{{variable}}`), also add `ProcessExpression()` calls in the preprocessors
-5. Write tests to verify the property survives the full rendering pipeline
+3. Add the property name to `KnownProperties.cs` for the corresponding element type (YAML validation)
+4. Add the property to `TemplateElement.CopyBaseProperties()` (single source of truth in `Parsing/Ast/TemplateElement.cs`)
+5. If the property contains expressions (e.g., `{{variable}}`), also add `ProcessExpression()` calls in the preprocessors
+6. Write tests to verify the property survives the full rendering pipeline
 
 ## Important Patterns
 

CLAUDE.md

@@ -26,6 +26,8 @@ When reviewing code changes, verify:
 - [ ] Null checks via `ArgumentNullException.ThrowIfNull()` -- not manual `if (x == null)`
 - [ ] Resource limits preserved -- never remove or weaken `MaxFileSize`, `MaxNestingDepth`, `MaxRenderDepth`
 - [ ] New element types follow the switch-based dispatch pattern (not base class properties)
+- [ ] New YAML properties registered in `KnownProperties.cs` for validation and typo suggestions
+- [ ] Tests using shared static state use `[Collection(..., DisableParallelization = true)]`
 - [ ] XML docs on all public API surface
 - [ ] Snapshot tests added/updated for visual changes (`UPDATE_SNAPSHOTS=true`)
 - [ ] Wiki pages updated -- if code changes affect public API, element properties, YAML syntax, builder methods, or CLI options, update the corresponding `docs/wiki/` pages