volk_prefs: add XDG Base Directory support

[mtibbits]

Summary

Add XDG Base Directory support to volk_get_config_path() and
volk_profile. New profiles are written to $XDG_CONFIG_HOME/volk/
(defaulting to ~/.config/volk/), with ~/.volk/ demoted to a
read-only fallback. When volk_profile detects a legacy config, it
offers an interactive migration prompt (suppressed in non-interactive
contexts).

GNU Radio already migrated to XDG (gnuradio/gnuradio#7136); this
aligns VOLK so both configs live in the same tree.

CONFIG vs CACHE

@jdemel noted that volk_config resembles FFTW wisdom
(regenerable, machine-specific profiling data), raising the question of
whether $XDG_CACHE_HOME would be more appropriate.

This PR uses $XDG_CONFIG_HOME because:

1. The issue specifically requests a config path change
2. GNU Radio's migration used $XDG_CONFIG_HOME
3. Users expect volk_config alongside GNU Radio's config
4. Cache data can be deleted without consequence; losing volk_config
   forces a full re-profile (~minutes), worse UX than typical cache loss

Happy to switch to $XDG_CACHE_HOME if the maintainers prefer —
the implementation is structurally identical.

Fallback chain

1. $VOLK_CONFIGPATH/volk/volk_config      (env override, unchanged)
2. $XDG_CONFIG_HOME/volk/volk_config      (NEW)
3. $HOME/.config/volk/volk_config         (NEW)
4. $HOME/.volk/volk_config                (legacy, read-only fallback)
5. $APPDATA/.volk/volk_config             (Windows, unchanged)
6. /etc/volk/volk_config                  (system-wide, unchanged)

Related issues

Fixes gnuradio#792

Testing

• Fresh install (no config): volk_profile writes to ~/.config/volk/volk_config
• Legacy config, accept migration: file moved, empty ~/.volk/ removed
• Legacy config, decline migration: file stays, profile writes there too
• Non-interactive (piped stdin): no prompt, keeps legacy path
• -p /tmp/custom override: no prompt, writes to custom path
• XDG_CONFIG_HOME=/tmp/xdg-test: writes to /tmp/xdg-test/volk/volk_config
• XDG_CONFIG_HOME= (empty): treated as unset per XDG spec
• Dual files exist: XDG path takes precedence on read
• Move fails (permissions): warning printed, falls back to legacy

Checklist

• Builds cleanly (cmake --build)
• Tests pass (ctest)
• Commits are signed off (git commit -s)
• PR does one thing — no unrelated changes mixed in