Add alpha PHOENIX support, workflow recipes, instrument readers, and example notebook#3

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ytsapras wants to merge 12 commits intoebachelet:masterfrom

ytsapras:feature/phoenix-backend

Commits on Mar 10, 2026

Improve PHOENIX fitter: optional RV init, scaling, subgrid selection
ytsapras
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Commits on Mar 20, 2026

Commits on Apr 8, 2026

feat: add XDG config support for PHOENIX path
ytsapras
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Commits on Apr 10, 2026