Feature request: add CLI option to disable specific tuners / graceful handling of disabled IPv6

When running bpftune with IPv6 disabled at kernel level (ipv6.disable=1 boot parameter), the neigh_table_tuner (and potentially other tuners) generate errors on startup because they try to access /proc/sys/net/ipv6/ paths that do not exist:

bpftune: could not open /proc/sys/net/ipv6/neigh/default/gc_interval (netns fd 0) for reading: No such file or directory
bpftune: could not open /proc/sys/net/ipv6/neigh/default/gc_stale_time (netfs fd 0) for reading: No such file or directory
bpftune: could not open /proc/sys/net/ipv6/neigh/default/gc_thresh1 (netns fd 0) for reading: No such file or directory
bpftune: could not open /proc/sys/net/ipv6/neigh/default/gc_thresh2 (netns fd 0) for reading: No such file or directory

This is a known situation also reported in #90.

Environment

• OS: Arch Linux
• Kernel: 6.18.13-xanmod1-1-x64v3
• CPU: AMD Ryzen Threadripper 3970X
• Boot parameter: ipv6.disable=1

Expected behavior

Bpftune should handle gracefully the case where IPv6 is disabled at kernel level, either by:

1. Option A (preferred): Detecting at startup that IPv6 is disabled (e.g. checking /proc/sys/net/ipv6 availability or CONFIG_IPV6 state) and skipping IPv6-related tunables silently, without logging errors.

2. Option B: Adding a CLI flag such as -D <tuner_name> (or --disable <tuner_name>) to allow the administrator to explicitly exclude specific tuners. This would also be useful for other scenarios where a tuner is not applicable to a given system configuration.

Start at bpftune startup and CLI parsing, then trace neigh_table_tuner accesses to /proc/sys/net/ipv6/. Review the related behavior in #90. Done means IPv6-disabled systems start without these errors, or administrators can reliably exclude the affected tuner with the chosen CLI option.