Closes #455

Summary

• Created docs/guides/COLLECTOR_DEVELOPMENT.md (1055 lines) covering all 7 sections from the issue specification
• All code examples derived from actual source code analysis of collector_base.h, collector_plugin.h, metric_factory.h, collector_adapters.h, and existing collector implementations
• Includes ASCII architecture diagrams, decision tables, and complete code walkthroughs

Sections

1. Collector Architecture - Dual interface overview (collector_base<T> CRTP vs collector_plugin), factory system diagram, interface comparison table, collection lifecycle
2. Implementing a Custom Collector - Step-by-step CRTP walkthrough, create_base_metric usage, configuration convention, alternative plugin approach, hybrid CRTP+plugin pattern
3. Platform-Specific Collectors - #ifdef guards vs Strategy pattern with decision table, platform detection macros, graceful fallback patterns, Windows include ordering
4. Thread Safety Requirements - Method-level safety requirements, built-in CRTP thread safety, atomic operations for high-frequency metrics, thread-local buffer integration, safe shutdown
5. Factory Registration - Three registration methods (helper functions, direct factory, REGISTER_COLLECTOR macro), instance creation APIs, plugin system registration, config-driven selection
6. Testing Custom Collectors - Unit test patterns (lifecycle, metrics, disabled state, statistics, health, types), factory tests, platform tests, performance benchmarks, thread safety tests
7. Reference: Built-in Collector Catalog - 17 collectors across 4 categories (core, utility, hardware plugin, container plugin) with files, metrics, and platform support

Test Plan

• CRTP pattern fully explained with diagrams
• Complete custom collector implementation walkthrough
• Platform-specific implementation patterns documented
• Thread safety requirements clearly stated
• Factory registration process documented
• Testing guide with examples
• Built-in collector reference table
• Verify all referenced source files exist in codebase (28/28 verified)
• Review consistency with existing documentation (terminology matches ARCHITECTURE.md, plugin_architecture.md)

Verification Notes

• File references: All 28 referenced paths verified (24 headers + 4 documents)
• Code accuracy: All code examples derived from actual source files, not hypothetical
• Naming consistency: Metric naming convention (<domain>.<metric_name>) consistent with existing collectors
• Interface accuracy: Both collector_base<T> and collector_plugin APIs match actual header signatures