to_full_markdown() — rename lossless implementation + tests

[DEVunderdog]

Parent

#268

What to build

Rename the current to_markdown() method on StructuralProfileResult to to_full_markdown(). The implementation is unchanged — this is a pure rename preserving the existing lossless behaviour. Add unit tests in tests/unit/profiling/ asserting that to_full_markdown() output is content-equivalent to to_dict(): every field present in to_dict() must appear in the Markdown output, including histogram bins, full correlation matrices, memory breakdown, and all per-column fields.

Acceptance criteria

• StructuralProfileResult.to_full_markdown() exists and produces output identical in content to the previous to_markdown()
• StructuralProfileResult.to_markdown() still exists (can be a stub or placeholder — will be implemented in a follow-on slice)
• Unit tests in tests/unit/profiling/ cover to_full_markdown() and assert losslessness against to_dict()
• Docstring on to_full_markdown() describes it as the lossless Markdown serialization for debugging and archival use

Blocked by

None — can start immediately