When kind-mount-root is set in spec.yml, emit a single extraMount
mapping the root to /mnt instead of per-volume mounts. This allows
adding new volumes without recreating the Kind cluster.
Volumes whose host path is under the root skip individual extraMounts
and their PV paths resolve to /mnt/{relative_path}. Volumes outside
the root keep individual extraMounts as before.
Cherry-picked from branch enya-ac868cc4-kind-mount-propagation-fix
(commits b6d6ad81, 929bdab8) and adapted for current main.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>