feat(spaces): implement Phase 3 Persistent Transclusion Context

Implements persistent transclusion context for Information Spaces:

- ScopedVariables: Variable scope layers (request > document > space)
- SpaceTransclusionContext: Extends TransclusionContext with DB persistence
- CrossSpaceResolver: Resolve references across space boundaries
- ReferenceGraph: Track document dependencies for cache invalidation
- PersistentReferenceGraph: Repository-backed reference tracking
- RenderCache: Cache rendered output with invalidation support
- CacheInvalidator: Event-driven cache invalidation using reference graph

Key features:
- Variable precedence: request overrides document overrides space
- Reference tracking during transclusion processing
- Transitive dependent calculation for cache invalidation
- Event bus integration for automatic invalidation on content changes

47 unit tests covering all components.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

markitect/spaces/transclusion/__init__.py

@@ -6,7 +6,40 @@ This package extends the existing TransclusionContext with:
 - Cross-space reference resolution
 - Reference graph for dependency tracking
 - Variable scope layers (space, document, request)
+- Event-driven cache invalidation
 """
 
-# Transclusion extensions will be implemented in Phase 3
-__all__ = []
+from .persistent_context import (
+    SpaceTransclusionContext,
+    ScopedVariables,
+    VariableScope,
+    CrossSpaceResolver,
+)
+from .reference_graph import (
+    ReferenceGraph,
+    PersistentReferenceGraph,
+    DependencyNode,
+)
+from .cache_invalidation import (
+    RenderCache,
+    CacheEntry,
+    CacheInvalidator,
+    create_invalidation_handler,
+)
+
+__all__ = [
+    # Persistent context
+    "SpaceTransclusionContext",
+    "ScopedVariables",
+    "VariableScope",
+    "CrossSpaceResolver",
+    # Reference graph
+    "ReferenceGraph",
+    "PersistentReferenceGraph",
+    "DependencyNode",
+    # Cache invalidation
+    "RenderCache",
+    "CacheEntry",
+    "CacheInvalidator",
+    "create_invalidation_handler",
+]