[MULTICAST FORWARDING] Multicast forwarding lacks receiver-aware decision making

### Current multicast forwarding in NuttX is local-membership driven rather than receiver-aware

- IPv4, forwarding is based on the device’s IGMP group list, performing a fan-out/relay for groups joined locally. [here](https://github.com/apache/nuttx/blob/master/net/devif/ipv4_input.c#L379)
- IPv6, multicast packets are just forwarded. [here](https://github.com/apache/nuttx/blob/master/net/devif/ipv6_input.c#L323)

### This issue discusses whether NuttX should evolve toward receiver-aware multicast forwarding

Such a change may involve:
- Control path: introducing IGMP/MLD snooping or similar mechanisms to learn downstream membership
- Data path: adding a multicast forwarding struct and corresponding forwarding logic
- User space: other potential support

From project history I found that NuttX has evolved to supporting multiple netdevs and basic forwarding, extending multicast forwarding may be a reasonable next step.

### On which OS does this issue occur?

[OS: Linux]

### What is the version of your OS?

Ubuntu 24

### NuttX Version

All version

### Issue Architecture

[Arch: all]

### Issue Area

[Area: Networking]

### Host information

_No response_

### Verification

- [x] I have verified before submitting the report.