feat(spaces): implement Phase 4 HTML Rendering Mode

Implements HTML rendering system for Information Spaces:

- SpaceRenderer: Abstract base class for renderers
- RenderConfig: Configuration for format, theme, TOC, etc.
- RenderResult: Immutable result with content hash and metadata
- ThemeConfig: Layered theme system with customization
- CompositeRenderer: Multi-format renderer delegation

- MarkdownToHTMLRenderer: Full markdown-to-HTML conversion
  - Theme support (github, dark, minimal, academic)
  - Code block handling
  - Link target="_blank" for external links
  - Table of contents generation
  - Heading ID generation for navigation
- HTMLRendererFactory: Factory for common renderer configurations

- SpaceRenderingService: Orchestration layer
  - Transclusion variable substitution
  - Render caching with automatic invalidation
  - Event emission (RENDER_STARTED, RENDER_COMPLETED, RENDER_FAILED)
  - Batch rendering support
  - Statistics tracking
- SpaceRenderingServiceBuilder: Fluent builder pattern

60 unit tests covering all components.

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

markitect/spaces/rendering/__init__.py

@@ -6,7 +6,42 @@ This package provides space rendering capabilities:
 - MarkdownToHTMLRenderer: HTML output renderer
 - Theme support and customization
 - Render caching with invalidation
+- SpaceRenderingService: Orchestration layer
 """
 
-# Rendering will be implemented in Phase 4
-__all__ = []
+from .base import (
+    SpaceRenderer,
+    RenderConfig,
+    RenderResult,
+    RenderFormat,
+    ThemeConfig,
+    CompositeRenderer,
+)
+from .html_renderer import (
+    MarkdownToHTMLRenderer,
+    HTMLRendererFactory,
+    THEME_PROPERTIES,
+    combine_theme_properties,
+)
+from .service import (
+    SpaceRenderingService,
+    SpaceRenderingServiceBuilder,
+)
+
+__all__ = [
+    # Base classes
+    "SpaceRenderer",
+    "RenderConfig",
+    "RenderResult",
+    "RenderFormat",
+    "ThemeConfig",
+    "CompositeRenderer",
+    # HTML renderer
+    "MarkdownToHTMLRenderer",
+    "HTMLRendererFactory",
+    "THEME_PROPERTIES",
+    "combine_theme_properties",
+    # Service
+    "SpaceRenderingService",
+    "SpaceRenderingServiceBuilder",
+]