[Request]: Automatic public IPv6 prefix mapping / delegation from host

### Description

Currently, the main way to expose containers to public IPv6 is through manual port publishing (`-p [::]:hostport:containerport`) or using `--network host`.

I would like **native support for automatically mapping the host's public IPv6 address** (Global Unicast Address) to containers, so that containers can directly receive and use the host's public IPv6 prefix without manual port mapping.

### Use Case

- Home users with native IPv6 broadband (/64 or /56 prefix from ISP).
- Running web services, APIs, or other servers that need to be directly accessible via public IPv6.
- Simplifying deployment by allowing containers to behave more like they have their own public IP, reducing port conflicts on the host.

### Proposed Feature

Add support for **automatic public IPv6 mapping**, for example:

```bash
# Option 1: When creating a network
container network create mynet --subnet-v6 auto-public

# Option 2: When running a container
container run --network mynet --ipv6-public-mapping ...

# Option 3: A global flag
container run --public-ipv6 ...

The issue names no files, tests, or entry points. Start by locating the existing network creation, IPv6, and port-publishing paths, then determine how automatic public-prefix delegation should integrate with them; done requires an agreed behavior and implementation scope for the proposed options.

Written by the indexing model from the issue text.