Move peer discovery into core-react-native

[gmaclennan]

Context

Milestone-doc nice-to-have: "Moving discovery into core-react-native".

Today peer discovery (mDNS / local-network) lives outside this module. Pulling it into core-react-native would let the FGS own the full discovery + sync lifecycle alongside the backend it already manages, rather than splitting it across the host app.

Scope (to refine)

• Inventory where discovery currently runs and what it depends on.
• Decide what moves into the native FGS layer vs. the Node backend.
• Handle Android local-network / nearby-devices permissions and the iOS local-network prompt.

Exploratory — needs a design pass before estimating.