Expose provider axis type metadata

[jc-macdonald]

Summary

• publish normalized axis_types beside provider axis order, sizes, coordinates, and labels
• distinguish categorical, ordinal, and continuous semantics without coordinate-value heuristics
• cover every normalized axis type and the compatibility subgroup axis
• document the public provider metadata surface

Validation

• just ci
  • 539 core tests passed, 16 skipped
  • 73 provider tests passed, 1 skipped
  • strict Ruff, mypy, and documentation builds passed

Closes #234
Unblocks ACCIDDA/op_engine#94.