#1272 swamp extension version crashes with 'Invalid CalVer version: "null"' when only a prerelease (beta/rc) version is published

Summary

swamp extension version throws Invalid CalVer version: "null" for an extension that has a prerelease version published (e.g. on the beta channel) but no stable version. It appears to feed the null stable latest-version straight into the CalVer parser instead of handling the no-stable case (falling back to the latest prerelease, or computing next from today's date).

Steps to reproduce

1. Publish an extension only to a prerelease channel: swamp extension push <manifest> --channel beta --yes (so the registry has latestBeta set but latestVersion/stable = null)
2. Ask for the next version: swamp extension version --manifest <manifest> --json

Actual

{
  "error": "Failed to look up version for @shrug/mercury: Invalid CalVer version: \"null\". Expected format [HOST-1] (e.g., \"2025.01.15.1\")"
}

swamp extension info @shrug/mercury --json confirms the state: "latestVersion": null, "latestBeta": "2026.07.18.3", "latestRc": null.

Earlier, before any publish (currentPublished null AND no prerelease), the same command returned a valid { currentPublished: null, nextVersion: "2026.07.19.1" } — so the null-stable case is only mishandled once a prerelease exists.

Expected

nextVersion should be computed even when only a prerelease exists — e.g. from the latest prerelease version and/or today's date — rather than passing the literal null stable version to the CalVer parser. At minimum, treat a null stable latestVersion as "no stable baseline" (as it already does when nothing is published at all).

Impact

Blocks the documented State 5 (versioned) publish-flow helper for any extension that follows the recommended beta→rc→stable promotion path: after a first beta publish you can no longer use swamp extension version to compute the next version. Workaround: bump the manifest version by hand.

Environment

• swamp 20260716.194319.0-sha.df9fc306
• Linux
• First surfaced with local extensions @shrug/{mercury,invoiceninja,bank-import} published to the beta channel.