Tenant / VRF context placeholder for per-tenant topology views

Track where per-tenant topology scoping will need to be added to lake when per-tenant colors are deployed. No UI change required now — add a code comment and this issue as a tracked placeholder.

**Milestone:** RFC-18: Observability
**RFC:** https://github.com/malbeclabs/doublezero/pull/3288

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**Repo:** lake
**Size:** Small

**Blockers:** #3358 (service-aware path visualization)