[Bug]: HostAgent silent packet drop when mapping explicit IPv6 hostIP without guestIP

# Description

## Summary:
Lima's port forwarding defaults are currently protocol-blind. When a user explicitly defines an IPv6 hostIP (e.g., ::1) but omits the guestIP, Lima defaults the guestIP to an IPv4 address (127.0.0.1). This creates a protocol mismatch in the HostAgent dialer, causing packets to be dropped and client connections to hang. Additionally, Lima fails to set GuestIPMustBeZero when the guestIP is the IPv6 unspecified address (::), breaking wildcard listener logic.

## Environment:
OS: Windows (WSL2 Driver)

Lima Version: limactl version 2.1.1

Networking: Default user-mode networking

## Steps to Reproduce:

Create a Lima instance with the following port forward configuration:

```yaml
portForwards:
- guestPort: 8080
hostIP: "::1"
hostPort: 8080
```

Start the VM:

```bash
limactl start ipv6.yaml
```

Inside the VM, start a server listening on IPv6:

```bash
python3 -m http.server 8080 --bind ::
```

From the Windows Host, attempt to connect via IPv6:

```bash
curl.exe -I -6 "http://[::1]:8080"
```

## Expected Behavior:
The connection should succeed, or if guestIP is left blank, Lima should default the guestIP to ::1 to match the IPv6 hostIP.

## Actual Behavior:
The curl command hangs indefinitely. limactl logs show that the HostAgent is attempting to dial an IPv4 guestIP (127.0.0.1) for an incoming IPv6 request, resulting in a silent failure/packet drop.

## Root Cause Analysis:
The issue lies in pkg/limayaml/defaults.go within the FillPortForwardDefaults function. Currently, the logic hardcodes 127.0.0.1 as the fallback for GuestIP regardless of the HostIP protocol. This prevents the HostAgent from establishing a valid TCP bridge when IPv6 is explicitly requested on the host side.