System paper: JOSS paper.md in-repo + IJM description article, at first fully-dossiered release

Task

The populace system paper, in two artifacts:

1. JOSS submission — paper.md in this repository (JOSS requires the paper in the software repo): the stack's architecture (Frame kernel + operator shards, typed weights, constellation versioning, weights-by-construction contracts), statement of need, and the validation program. Fast, DOI-citable, the launch citation for the software itself.
2. IJM description article — the EUROMOD-description analog: architecture, the certified-release program, and the validation stack, citing the operator dossiers (l0-paper, imputation-paper, calibration-paper), the popdgp referee, the composition study, and the release-dossier machinery (#302, populace-benchmarks#6).

Timing: at launch — after the first certified release built from the current spine (ASEC 2024/25, #296) with auxiliaries (#297) publishes its full validation dossier. The Belgium cross-engine note (#303) folds in here if not standalone.

🤖 Generated with Claude Code

Start with the repository's proposed paper.md and the first certified-release validation dossier after #296 and #297 publish. Cover the architecture, statement of need, validation program, certified-release process, and cited operator dossiers, including #302 and populace-benchmarks#6; fold in #303 if it is not standalone. Done means both the JOSS paper and IJM description article are complete and launch-ready.