Add pair-level relation tracking modifiers

Normalized profile

What this task asks for

Tracking modifiers should distinguish changes to specific relation pairs, not just trait-level additions and removals. Tracking modifiers detect trait-level additions and removals but cannot distinguish which specific relation pair changed, blocking per-target reactivity. Make tracking modifier factories accept new `RelationPair`. The target `'*'` acts as a wildcard. Pair modifiers compose with `Or`. Pair modifiers combined with regular trait parameters in the same query must satisfy all constraints together. The `entity.changed` method accepts a `RelationPair` for manual pair-level change signaling.