ci(release): stack inventory rejects a patch that pins a chart released after the branch cut #2972

Problem

The tag release step renders each stack release's inventory with docs-version-sync. For every source chart the stack pins, it requires the chart's release tag to be an ancestor of the stack commit. After #2971, it also accepts an ancestor of the default-branch commit that a release branch snapshot copies.

A patch on a stack release branch that moves a pin to a chart version released after the branch cut still fails that check. The newer chart tag comes from the default branch or from a chart maintenance branch, so it is not in the stack's history. The patch release is then published without its inventory. This applies to every stack train, including v1.0.

Expected

• A stack patch can pin any released version of a source chart, and its inventory still records the chart's source tag and commit.
• A tag that is not a release of that chart is still rejected.