feat(dsl): add .array() method for type-safe array creation in workflow steps

Implement a new .array() method in the Flow class to enable type-safe, semantic array
steps that leverage existing validation and dependency mechanisms.
The change includes
type constraints to enforce array return types, comprehensive runtime and type tests,
and detailed documentation.
This enhancement maintains full feature parity with .step()
and integrates seamlessly with existing validation utilities, laying the foundation for
future phases involving queue routing and advanced type inference.
The implementation
follows the principle of minimal, non-breaking extension, ensuring zero risk to current
functionality and providing immediate compile-time safety benefits.

Author: jumski

.changeset/tricky-sites-stare.md

@@ -0,0 +1,17 @@
+---
+'@pgflow/dsl': minor
+---
+
+Add `.array()` method for type-safe array step creation
+
+Introduces a new `.array()` method that provides compile-time type safety for array-returning handlers with zero runtime overhead.
+
+- Enforces array return types at compile time
+- Pure delegation to existing `.step()` method
+- Full support for dependencies and runtime options
+- Backward compatible
+
+```typescript
+flow.array({ slug: 'items' }, () => [1, 2, 3]);  // ✅ Valid
+flow.array({ slug: 'invalid' }, () => 42);       // ❌ Compile error
+```