feat(themis): implement complete course XML export with embedded module specifications

Implements Themis 2.3 - XML Export Functionality, completing Themis MVP (100%)

FEATURES:
- Complete course XML export matching courseSchema.xml hierarchy
- Embeds full module specifications from moduleData.xmlContent
- Pre-download validation with courseValidator.ts integration
- User feedback via success/error banners with collapsible details
- Export XML button in CourseOverview with disabled state until complete
- Proper XML indentation and structure (2-space hierarchy)
- Temporal metadata and generation provenance tracking

IMPLEMENTATION:
- src/lib/utils/validation/outputSerialiser.ts
  * Complete rewrite (448 lines)
  * serialiseCourseToXml() generates complete XML
  * serializeModule() embeds module specifications
  * extractModuleSpecificationContent() removes duplicate <Module> tags
  * indentXml() maintains consistent formatting
  * Includes metadata: timestamps, model info, generation source

- src/lib/components/themis/CourseOverview.svelte (+127 lines)
  * Export XML button with validation handler
  * handleXmlExport() validates before download
  * Success banner with auto-hide (5 seconds)
  * Error banner with collapsible error/warning lists
  * Close button for manual dismissal
  * CSS styling for banners and validation feedback

DOCUMENTATION:
- docs/dev/features/course-xml-export.md (400 lines)
  * Feature overview and technical implementation
  * Usage instructions and file format details
  * Validation requirements and error handling
  * Integration points and future enhancements

ROADMAP UPDATES:
- Themis 2.3 marked complete in Themis-MVP.md
- Themis status updated to 100% MVP in README.md
- Recent wins updated with XML export achievement
- Next milestones reordered (Dark Mode now #1)

VALIDATION:
- Validates against courseValidator.ts before download
- Blocks download on errors, allows on warnings
- Displays detailed feedback to user
- Ensures exported XML meets quality standards

THEMIS MVP STATUS: ✅ 100% Complete
All milestones (2.1 Foundation, 2.2 Schema, 2.3 Export) now complete

Files changed: 6
- outputSerialiser.ts: 448 lines (complete rewrite)
- CourseOverview.svelte: +127 lines
- course-xml-export.md: 400 lines (new)
- Roadmap updates: 3 files

Closes #themis-2.3

Author: JasonWarrenUK