feat: expose interface hardware (MAC) address

[franchb]

Summary

Adds a mac_addr: Option<MacAddr> field and an Interface::mac_addr() accessor that report an interface's 6-byte hardware (MAC) address.

The address is read from the same OS interface-enumeration call already used for the IP information — no reaching into /sys or other platform-specific locations:

• Linux/Android — link-layer getifaddrs entries (AF_PACKET / sockaddr_ll)
• BSD / Apple / illumos — AF_LINK / sockaddr_dl
• Windows — GetAdaptersAddresses PhysicalAddress

MacAddr wraps [u8; 6] with:

• Display formatting as aa:bb:cc:dd:ee:ff
• octets() and is_zero() helpers
• From<[u8; 6]>

Only 6-byte (EUI-48) addresses are reported; other lengths (e.g. InfiniBand) yield None.

sockaddr_dl correctness (BSD/Apple)

sockaddr_dl is a variable-length structure: the interface name (sdl_nlen bytes) is followed by the link-layer address starting at sdl_data. Since libc models sdl_data as a fixed-size minimum work area (only [c_char; 12] on Apple), the MAC is read via pointer arithmetic from the start of sdl_data and bounded by the structure's own sdl_len (on platforms that have it; illumos/Solaris lack sdl_len and are excluded). This avoids dropping the address for interfaces whose name is 6+ characters (e.g. bridge0). addr_of!/offset_from are used to respect the crate's 1.71 MSRV.

Testing

• test_get_if_addrs now cross-checks the parsed MAC against ip addr / ifconfig output, asserting the crate reports a MAC whenever the system tool does.
• New test_mac_addr_display covers MacAddr formatting and helpers.
• Verified green in CI on Linux, macOS (real runtime coverage of the sockaddr_dl path) and Windows, plus cross-builds for Android, illumos, FreeBSD, NetBSD and the Apple tier-3 targets.

Notes

• This bumps the version to 0.16.0; happy to drop that commit/line if you'd prefer to manage the version bump yourself.

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