Segment membership: Property dependency model

[khvn26]

We'll need to adapt Segment membership data model to free JSONPath context value evaluation, should we decide to implement it.

Scope

• Atoms carry dependency flags derived once at atom creation by a JSONPath classifier.
• Maintenance router walks dependency flags rather than the property string.
• JSONPath canonicalisation in atom keys.

Acceptance criteria

• A trait write to profile.address.city only re-evaluates atoms whose dependencies include profile.
• $.identity.traits.country and $.identity['traits']['country'] collapse to the same atom.