broker: replace name-keyed StoreSync with Zone-qualified v3 identity

[vicondoa]

Problem

The v3 Guest setup-Volume path currently reuses the legacy StoreSync request keyed by vm_id/Guest name and a name-keyed store-view intent. U20 needs this compatibility path to synchronize the controller-owned Guest closure, but it is not sufficient for same-named Guests across Zones or reincarnation fencing.

Required follow-up

• Add a typed v3 StoreSync identity carrying Zone UID, Guest UID/generation, and the exact setup Volume UID/revision or equivalent sealed ownership proof.
• Resolve the store-view intent by Zone-qualified immutable identity rather than Guest name.
• Preserve broker-only path resolution and the single repair owner invariant.
• Remove the name-keyed compatibility path after all callers migrate.
• Cover same-name cross-Zone Guests, reincarnation, restart adoption, and foreign ownership refusal.

This is non-P0/P1 legacy cleanup discovered while completing U20 host integration; it does not block the single-Zone acceptance fixture.

[vicondoa]

Status check (2026-09-13, after PR #517 merged): still open. StoreSyncRequest is unchanged - it is still keyed by vm_id plus bundle_closure_ref/`generation_token` (packages/d2b-contracts-broker/src/broker_wire.rs:2645-2648) with no Zone UID / Guest UID / setup-Volume identity, so the name-keyed path this issue asks to replace is still the wire shape. (The unrelated consumerless OpenZoneStore op was retired in PR #521; this is a different request.)