* refactor(utils): extract common buffer utilities to detail namespace

- Extract calculate_percentile to detail namespace for reuse
- Extract ring_buffer_index helper for consistent buffer indexing
- Extract calculate_basic_statistics for common statistics computation
- Refactor time_series_buffer to use detail utilities
- Refactor load_average_history to use detail utilities

This reduces code duplication between time_series_buffer and
load_average_history classes, following Kent Beck's "No Duplication"
principle as part of C++20 module preparation.

Part of #309

* docs(utils): mark internal buffer APIs with @internal tag

Add @internal doxygen tags to internal buffer implementation files:
- ring_buffer.h
- buffering_strategy.h
- buffer_manager.h

This clarifies the public vs internal API boundary for C++20 module
preparation and helps developers understand which APIs are stable.

Part of #309

* docs: clarify public vs internal buffer APIs

Update architecture documentation to distinguish between:
- Public API: time_series_buffer, thread_local_buffer
- Internal API (@internal): buffer_manager, ring_buffer, buffering_strategy

Also add src/utils internal files to PROJECT_STRUCTURE.md to document
the internal buffer implementation layer.

Part of #309

* fix(adapters): replace deprecated common_system helper functions with member methods

Replace deprecated kcenon::common helper functions with Result member methods
to fix MSVC build warning treated as error (C4996):
- is_error(result) -> result.is_err()
- is_ok(result) -> result.is_ok()
- get_value(result) -> result.value()
- get_error(result) -> result.error()

Affected files:
- thread_system_adapter.h
- common_monitor_adapter.h
- common_system_adapter.h

* refactor(examples): update Result pattern usage to modern API

Update examples to use Result member methods instead of deprecated
helper functions for consistency with the codebase:
- result.is_ok() instead of is_ok(result)
- result.is_err() instead of is_error(result)
- result.value() instead of get_value(result)
- result.error() instead of get_error(result)

Also fix bool conversion: if(result) -> if(result.is_ok())

* docs(changelog): add MSVC build fix entry

Document the deprecated common_system helper function replacement that
fixes the MSVC build error (C4996 warning treated as error).