* feat(result): add consolidated headers for C++20 module preparation

Create three new consolidated headers following Kent Beck's
"Fewest Elements" principle:

- core.h: Combines fwd, error_info, result_core, and optional
- utilities.h: Combines result_funcs, exception_conversion, and macros
- compat.h: Contains error codes compatibility layer

This reduces header count from 9 to 3 logical files, preparing
for future C++20 module migration.

Refs: #255

* refactor(result): update umbrella header to use consolidated headers

Update result.h to include new consolidated headers (core.h,
utilities.h, compat.h) instead of individual headers.

This maintains backward compatibility while simplifying
the include structure.

Refs: #255

* refactor(result): deprecate individual headers with pragma message

Add deprecation warnings to old individual headers:
- fwd.h -> core.h
- error_info.h -> core.h
- result_core.h -> core.h
- optional.h -> core.h
- result_funcs.h -> utilities.h
- exception_conversion.h -> utilities.h
- result_macros.h -> utilities.h
- error_codes_compat.h -> compat.h

Headers still work but emit compiler warnings directing users
to the new consolidated headers. Original content preserved
for reference within #if 0 blocks.

Refs: #255

* refactor(concepts): update callable.h to use core.h

Replace deprecated fwd.h include with core.h in callable.h
to eliminate deprecation warnings.

Refs: #255

* docs: add Result<T> header consolidation to documentation

Update DEPRECATION.md with:
- New section for deprecated individual Result headers
- Migration guide from 8 headers to 3 consolidated headers
- Future C++20 module structure information

Update CHANGELOG.md with:
- Consolidated headers change entry
- Deprecation section for individual headers

Refs: #255