feat: add kind-mount-root for unified Kind extraMount

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>

stack_orchestrator/constants.py

@@ -46,3 +46,4 @@ runtime_class_key = "runtime-class"
 high_memlock_runtime = "high-memlock"
 high_memlock_spec_filename = "high-memlock-spec.json"
 acme_email_key = "acme-email"
+kind_mount_root_key = "kind-mount-root"