Merge dev into main: Gaia WCS robustness, relative calibration, sky masking, JOSS paper, docs

[charliekilpatrick]

Summary

Merges dev into main with pipeline hardening, calibration improvements, documentation and paper updates, and expanded unit tests.

Highlights

WCS / Gaia alignment (solve_wcs.py)

• Fixed iterative Gaia–SExtractor matching so Gaia coordinates are not collapsed across iterations.
• Added structured logging (in-frame Gaia counts, SExtractor counts, match summaries, WCS diagnostics).
• Graceful fallback to coarse astrometry when Gaia refinement fails or is unstable; optional GAIAFAIL header and dispersion defaults so frames are not dropped silently.
• WCS sanity checks (scale vs instrument expectation, anisotropy) to reject pathological refined fits before reprojection.

Sky & stacking (instrument.py, image_procs.py)

• Iterative sigma-clipped sky creation and configurable masking for master sky; robust sky scaling in process_science.
• Optional relative flux calibration before stacking (--relative-calibration): SExtractor catalogs, RA/Dec cross-matching, FLUXERR_AUTO weighting; stack_data uses relative scales without dividing by exptime when provided; per-frame fallback 1/exptime when matching fails.

Photometry / utilities

• run_sextractor optional sextractor_path; find_catalog and 2MASS K-band mapping fixes as applicable.

Paper & docs

• JOSS paper.md updates (Summary, Statement of need, State of the field, Software design, Research impact, YAML affiliations, word count within limits).
• README / CONTRIBUTING / installation docs as on dev.

Tests & CI

• Unit tests for compute_relative_scales, create_sky, Gaia fallback paths, etc.; integration tests marked and excluded from default CI.

Checklist

• CI green on dev
• Docs / GitHub Pages deploy policy matches main (if applicable)
• Tag release after merge if desired

Suggested reviewers

Core maintainers familiar with solve_wcs, image_procs, and release process.