feat: add FIB lookup user plane metrics

[gruyaume]

Description

Improve user plane observability by adding two metrics::

Metric	Description	Type
app_xdp_fib_lookup_total	FIB lookup outcomes in the XDP data plane, with labels for interface (n3, n6) and result matching kernel return codes (success, no_neigh, blackhole, unreachable, prohibit, no_src_addr, frag_needed, not_fwded, fwd_disabled, unsupp_lwt).	Counter
app_xdp_ifindex_mismatch_total	Packets dropped because the FIB-resolved interface did not match the expected N3/N6 interface, with label for interface (n3, n6).	Counter

We also fix an issue where the xdp code unconditionally overwrote the ifindex returned by bpf_fib_lookup with the hardcoded n6_ifindex or n3_ifindex. Now it compares fib_params->ifindex against the expected interface. On mismatch, it drops the packet and increment an ifindex_mismatch counter in route_stat.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules