Summary

Consolidate load_average_history class with time_series_buffer<T> template to eliminate code duplication.

Parent Issue

Part of #309 (Consolidate buffering layer - C++20 Module preparation)

Current State

• time_series_buffer.h contains two nearly identical implementations:
  • time_series_buffer<T>: Generic time-series buffer for arithmetic types
  • load_average_history: Specialized buffer for load average (3 double values)

Problem

• Both classes share ~90% identical code structure (ring buffer logic, statistics calculation)
• Violates Kent Beck's "No Duplication" principle
• Makes maintenance harder - bug fixes must be applied twice

Proposed Solution

1. Create multi_value_time_series_buffer<T, N> template or similar for multi-field samples
2. Refactor load_average_history to use the new template or a specialized time_series_buffer
3. Mark old API as deprecated (if breaking changes not allowed)

Acceptance Criteria

• Single ring buffer implementation for time-series data
• load_average_history functionality preserved
• system_resource_collector continues to work
• Tests pass
• No breaking changes to public API (use deprecation if needed)

Files Affected

• include/kcenon/monitoring/utils/time_series_buffer.h
• include/kcenon/monitoring/collectors/system_resource_collector.h
• tests/test_time_series_buffer.cpp