Add arXiv preprint badge and citation metadata

The paper is published as arXiv:2605.19850
(https://arxiv.org/abs/2605.19850).

- README.md: arXiv badge alongside build / license / DOI badges.
- README.md Citation section: an @Article entry for the arXiv
  preprint (the form a reader citing the work should use), with
  the existing @misc Zenodo entry retitled and kept as the
  cite-the-code/data option.
- CITATION.cff: a preferred-citation block of type article
  pointing at the arXiv paper, so GitHub's "Cite this repository"
  button surfaces the paper rather than the software record. The
  stale placeholder comment about unminted DOIs is replaced.

Author: djankov

CITATION.cff

@@ -1,6 +1,7 @@
 # Machine-readable citation for GitHub's "Cite this repository" button.
-# DOI / version / release date are placeholders until the v0.1.0
-# release-cut PR (#8) mints the Zenodo concept DOI and tags the release.
+# The top-level metadata describes the software / data release archived
+# on Zenodo; `preferred-citation` points at the arXiv paper, which is
+# what a reader citing the work should use.
 cff-version: 1.2.0
 message: "If you use this work, please cite it using the metadata below."
 title: "How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation"
@@ -23,3 +24,13 @@ keywords:
 date-released: "2026-05-18"
 version: "0.1.0"
 doi: "10.5281/zenodo.20277028"
+preferred-citation:
+  type: article
+  title: "How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation"
+  authors:
+    - family-names: Jankovic
+      given-names: Dimitrije
+      affiliation: "Independent researcher — astro-tools"
+  year: 2026
+  doi: "10.48550/arXiv.2605.19850"
+  url: "https://arxiv.org/abs/2605.19850"

README.md

@@ -3,6 +3,7 @@
 [![build](https://github.com/astro-tools/paper-tle-divergence-atlas/actions/workflows/build.yml/badge.svg)](https://github.com/astro-tools/paper-tle-divergence-atlas/actions/workflows/build.yml)
 [![License: MIT](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.20277028.svg)](https://doi.org/10.5281/zenodo.20277028)
+[![arXiv](https://img.shields.io/badge/arXiv-2605.19850-b31b1b.svg)](https://arxiv.org/abs/2605.19850)
 
 > *How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation.*
 
@@ -72,17 +73,34 @@ scratch (different corpus, or just a clean slate), delete
 
 ## Citation
 
+To cite the paper, use the arXiv preprint:
+
+```bibtex
+@article{jankovic_tle_divergence_atlas_2026,
+  author        = {Jankovic, Dimitrije},
+  title         = {How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation},
+  year          = {2026},
+  eprint        = {2605.19850},
+  archivePrefix = {arXiv},
+  primaryClass  = {astro-ph.EP},
+  doi           = {10.48550/arXiv.2605.19850},
+  url           = {https://arxiv.org/abs/2605.19850}
+}
+```
+
+To cite the sweep outputs and source code specifically, use the Zenodo deposit:
+
 ```bibtex
 @misc{jankovic_tle_divergence_atlas,
   author = {Jankovic, Dimitrije},
-  title = {How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation},
+  title = {How long can you trust a Starlink TLE? --- sweep outputs and source code},
   year = {2026},
   doi = {10.5281/zenodo.20277028},
   url = {https://doi.org/10.5281/zenodo.20277028}
 }
 ```
 
-The DOI above is the Zenodo concept DOI and resolves to the latest version. The `CITATION.cff` machine-readable equivalent powers GitHub's "Cite this repository" button.
+The Zenodo DOI is the concept DOI and resolves to the latest version. The `CITATION.cff` machine-readable metadata powers GitHub's "Cite this repository" button and names the arXiv paper as the preferred citation.
 
 ## Methodology — quick reference