Add Harker and TAS diagrams

[gronbow]

Summary

• add validated major-element input inspection for flat or unambiguous transposed CSV, TXT, and XLSX tables
• add customizable multi-panel Harker diagrams with smart layout, group styling, clean axes, and publication export bundles
• add guarded volcanic TAS classification using a versioned Le Maitre / Le Bas field asset and explicit boundary-review states
• formalize GeoSkills v0.3.0 documentation, Codex installation guidance, release status, and changelog

The reviewed spider-diagram work from #1 is now part of main; this PR contains the remaining Harker, TAS, and v0.3.0 release changes.

Scientific safeguards

• require explicit wt% or ppm units and preserve below-detection-limit values as missing
• do not add default Harker regressions or infer petrogenetic processes; warn when specialist lithologies are mixed
• require volcanic applicability confirmation and a declared composition basis before TAS classification
• preserve ambiguous TAS fields, boundary points, and out-of-scope points as explicit review states
• keep plotting deterministic and local; private inputs and generated outputs remain excluded from Git

Validation

• pytest -q: 69 passed
• quick_validate.py: Skill is valid
• Python compilation and Git whitespace checks passed
• Harker plotting tested with locally held published data; TAS tested with the committed synthetic fixture
• export QA confirmed embedded PDF fonts, editable SVG text, 600 dpi PNG/TIFF, and LZW TIFF compression
• Ubuntu and Windows CI passed on Python 3.11 and 3.12 after the final release update and base retarget