Compile grants with "via" field into FilterRules that are placed only
on nodes matching the via tag and actually advertising the destination
subnets. Key behavior:
- Filter rules go exclusively to via-nodes with matching approved routes
- Destination subnets not advertised by the via node are silently dropped
- App-only via grants (no ip field) produce no packet filter rules
- Via grants are skipped in the global compileFilterRules since they
are node-specific
Reduces grant compat test skips from 41 to 30 (11 newly passing).
Updates #2180
54db47badc