Closes #334

Summary

This PR adds Part 1 of the comprehensive cross-system integration guide, covering:

• Ecosystem dependency map: Visual tree showing all 7 kcenon systems (common, thread, container, logger, monitoring, database, network)
• Tier-based initialization order: Clear hierarchy from Tier 0 (foundation) to Tier 3 (integration)
• Dependency rationale: Explains why each system depends on others and how circular dependencies are mitigated
• Unified bootstrapper patterns: Complete examples using unified_bootstrapper for automatic system initialization
• Manual adapter wiring: Examples of typed_adapter, interface_adapter, and cross-system adapters
• Service container integration: DI patterns for singleton, transient, and instance registration
• Lifecycle management: Complete application lifecycle from configuration loading to graceful shutdown

Changes Made

• Created docs/INTEGRATION_GUIDE.md (604 lines)
• Documented all 7 systems with their tier levels and key interfaces
• Provided complete code examples for both bootstrapper and manual wiring approaches
• Explained shutdown sequence and timeout handling
• Added references to related documentation

Test Plan

• Document builds correctly (Markdown syntax valid)
• Code examples compile (verified against existing headers)
• All 7 systems referenced accurately (cross-checked with ARCHITECTURE.md)
• Links to related docs are correct
• Tier dependencies match DEPENDENCY_MATRIX.md

Next Steps

Part 2 and Part 3 will be implemented in separate PRs:

• Part 2 ([Task] docs: Integration guide - Part 2: Integration scenarios and configuration #335): Integration scenarios and configuration management
• Part 3 ([Task] docs: Integration guide - Part 3: Error handling, lifecycle, and complete example #336): Error handling, complete example, CMakeLists.txt setup