design-proposal: revise public-IP resource proposal per review

Rework the stub in response to review and maintainer steers:

- Rename the kinds PublicIP/PublicIPClaim/PublicIPClass -> IPAddress/
  IPAddressClaim/IPAddressClass. A class names a pool, and pools are not
  necessarily public; the resource is the general address.
- Drop the "regression, not a wishlist" framing entirely. Cozystack uses
  kube-ovn only for subnets and VPCs, never its EIP objects, so there is no
  prior reserve-then-associate capability to preserve. Argue on merits: hold,
  move, quota, enumerate, bill.
- Add a Positioning section: this is not Cozystack-specific and belongs in a
  standalone, reusable project; Cozystack becomes one consumer. API group is a
  placeholder (ipam.example.com), deliberately not cozystack.io.
- Add Design section 7, worked examples against MetalLB, Cilium LB-IPAM, and a
  cloud provider, including the "plain LB Service, no annotations" collision
  case: reserved pools are autoAssign:false (or selector-gated) and disjoint
  from auto-assign pools, so a plain Service can never draw a reserved address.
- Security: specify controller identity by ServiceAccount and an allowlist for
  GitOps/system principals so admission does not break reconciliation.
- Status schema is a list (dual-stack), addressing the scalar-field gap.
- Defer multiple platform-owned addresses per workload as out of scope, with a
  note that the case may not be real (a router/VPN VM's extra addresses live on
  a tunnel the platform neither owns nor sees).
- Add a Naming note on the IPAddress overlap with core (KEP-1880) and Cluster
  API IPAM.

Assisted-By: Claude Opus 4.8
Signed-off-by: Timofei Larkin <lllamnyp@gmail.com>

Author: lllamnyp