routes: add health dimension to HA primary route election

Track unhealthy nodes in PrimaryRoutes so primary election skips them.
When all nodes for a prefix are unhealthy, keep the first as a degraded
primary rather than dropping the route entirely.

Anti-flap is built in: a recovered node becomes standby, not primary,
because updatePrimaryLocked keeps the current primary when still
available and healthy.

Updates #2129
Updates #2902