Handle building group deletion: clean up on-chain policies correctly

[martinjms]

Problem

When a building group is deleted from the DApp, only localStorage is cleaned up. The on-chain policies on those assemblies remain unchanged.

Additionally, if an assembly belongs to multiple building groups, deleting one group should only remove that group's rules from the assembly's policy — not wipe the entire policy.

Current behavior

• Delete building group → removed from localStorage only
• On-chain policies unchanged
• If assembly is in group A (rules 1,2) and group B (rules 3,4), Apply from group B overwrites all rules (loses rules 1,2)

Required behavior

1. On delete: remove only the rules associated with the deleted group from each assembly's on-chain policy
2. On Apply: merge rules from all building groups that contain the assembly
3. Exclusive check: warn if adding an assembly that's already in another group, or support multi-group membership with proper merging
4. Condition cleanup: condition objects can stay on-chain (they're reusable shared objects)

Design considerations

• Each assembly has ONE policy on-chain (flat rule list)
• Multiple building groups may reference the same assembly
• Need to track which rules came from which group, either in localStorage metadata or on-chain
• Consider making building group membership exclusive (simpler) vs supporting overlap (more flexible)

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