M3 Region migration

[Rikarin]

docs/plan/24 § Phase 4 · region migration · M3.

Moving a tenant, or a resource, between regions.

⚠ Blocked on two open questions rather than on effort — both in docs/plan/25, both tracked in #20:

• row 6, how many regions at GA. The default is one plus region migration at M3, which makes this row the thing that makes the default survivable.
• row 9, is a tenant one region forever, or is residency per-resource. The default is per tenant, and the note says per-resource residency is a much larger model. This row's size is decided entirely by that answer.

The topology already anticipates it: one regional control plane per region plus a single global directory, and CyberCloud.Tenancy holds the shard map a migration would rewrite.