Add trace-element spider diagrams

[gronbow]

What changed

• add validated primitive-mantle and N-MORB normalized trace-element spider diagrams
• support flat CSV/TXT/XLSX tables and unambiguous transposed supplement tables
• add explicit K2O, P2O5, and TiO2 wt% to elemental ppm conversions
• preserve missing and below-detection-limit values as gaps
• export SVG, PDF, 600 dpi PNG/TIFF, normalized source data, and JSON reports
• add versioned Sun and McDonough (1989) reference assets and method documentation

Scientific and user impact

GeoSkills can now produce reproducible, submission-oriented spider diagrams while preserving the stable REE workflow. The source-footnote modified primitive mantle is the default; the printed Table 1 primitive mantle and N-MORB remain explicit options. Ambiguous units, duplicate mappings, and non-positive logarithmic values block plotting rather than being guessed.

All processing remains local. Private input data and generated output directories are excluded from Git.

Validation

• quick_validate.py: Skill is valid
• pytest -q: 41 passed
• real published-data layout tested locally
• PDF size/font embedding, editable SVG text, 600 dpi PNG/TIFF, and LZW TIFF compression audited
• git diff --check: passed