Allocate a new floating IP

Allocates a public IPv4 address from an external network pool into the active scope, driven by the the floating IP creation form submission. Inputs mirror the visible form: region (fixed to the active scope's region), allocation pool, and an optional immediate association target. Floating IPs are billed pay-as-you-go only: billingCycle may be omitted (it defaults to pay-as-you-go). On acceptance the address becomes discoverable in the scoped list; when associateTo is omitted the address is created unassociated (status DOWN) and billed at the same rate as an associated one. Validation: scope must be active and writable by the caller (contributor or higher) → 403 FORBIDDEN otherwise; pool must name an existing external pool → 422 VALIDATION_FAILED; billingCycle, when given, must be pay-as-you-go → monthly / yearly are refused 422 VALIDATION_FAILED; associateTo, when present, must reference an instance/port reachable in the scope → 404 NOT_FOUND; floating-IP quota for the subscription must not be exhausted → 409 QUOTA_EXCEEDED; pool address exhaustion → 409 CAPACITY_UNAVAILABLE. Async & state: Asynchronous: returns 202 with the FloatingIp resource in its transitional state (status DOWN, fixedIp/instance/port/router null until association completes). The backend emits resource.updated SSE frames (kind: network.floating_ip, patch: {status, fixedIp, instance, port, router}) as allocation and the optional association settle; terminal states are ACTIVE (associated) or DOWN (allocated, unassociated). Errors: 401 unauthenticated; 403 missing create permission; 404 unknown associate target; 409 QUOTA_EXCEEDED when the scope's floating-IP quota is reached; 409 CAPACITY_UNAVAILABLE when the pool has no free addresses; 422 invalid body; 429 rate limited; 500 internal failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the original 202 result with the same FloatingIp id instead of allocating a second address.