feat: add Avalonia Zafiro development, layout, and viewmodel skills

skills/avalonia-zafiro-development/avalonia-reactive-rules.md

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+# Avalonia, Zafiro & Reactive Rules
+
+## Avalonia UI Rules
+
+- **Strict Avalonia**: Never use `System.Drawing`; always use Avalonia types.
+- **Pure ViewModels**: ViewModels must **never** reference Avalonia types.
+- **Bindings Over Code-Behind**: Logic should be driven by bindings.
+- **DataTemplates**: Prefer explicit `DataTemplate`s and typed `DataContext`s.
+- **VisualStates**: Avoid using `VisualStates` unless absolutely required.
+
+## Zafiro Guidelines
+
+- **Prefer Abstractions**: Always look for existing Zafiro helpers, extension methods, and abstractions before re-implementing logic.
+- **Validation**: Use Zafiro's `ValidationRule` and validation extensions instead of ad-hoc reactive logic.
+
+## DynamicData & Reactive Rules
+
+### The Mandatory Approach
+
+- **Operator Preference**: Always prefer **DynamicData** operators (`Connect`, `Filter`, `Transform`, `Sort`, `Bind`, `DisposeMany`) over plain Rx operators when working with collections.
+- **Readable Pipelines**: Build and maintain pipelines as a single, readable chain.
+- **Lifecycle**: Use `DisposeWith` for lifecycle management.
+- **Minimal Subscriptions**: Subscriptions should be minimal, centralized, and strictly for side-effects.
+
+### Forbidden Anti-Patterns
+
+- **Ad-hoc Sources**: Do NOT create new `SourceList` / `SourceCache` on the fly for local problems.
+- **Logic in Subscribe**: Do NOT place business logic inside `Subscribe`.
+- **Operator Mismatch**: Do NOT use `System.Reactive` operators if a DynamicData equivalent exists.
+
+### Canonical Patterns
+
+**Validation of Dynamic Collections:**
+```csharp
+this.ValidationRule(
+        StagesSource
+            .Connect()
+            .FilterOnObservable(stage => stage.IsValid)
+            .IsEmpty(),
+        b => !b,
+        _ => "Stages are not valid")
+    .DisposeWith(Disposables);
+```
+
+**Filtering Nulls:**
+Use `WhereNotNull()` in reactive pipelines.
+```csharp
+this.WhenAnyValue(x => x.DurationPreset).WhereNotNull()
+```