adblock: update 4.5.6-2

adblock: update 4.5.6-2

- add f_mem() helper to read MemAvailable from /proc/meminfo,
  replacing three duplicated inline reads in f_load, f_dns and f_jsnup
- cap the auto-detected CPU core count by available memory in f_load:
  adb_cores is limited to MemAvailable / 48 (MiB per job), floored to at least 1 core;
  this bounds the number of feeds processed in parallel on constrained devices,
  a user-set adb_cores is still honored as a ceiling (the cap only ever lowers it).
- derive the GNU sort buffer size from available memory
- readme update:
  - added a "low memory systems" recommendation to use
    the zram-swap package (compressed swap) with a swappiness hint,
    plus a sizing rule of thumb — this works for all DNS backends OOTB
  - significantly expanded the custom-feeds section (all JSON fields, rule parameters, etc.)

Signed-off-by: Dirk Brenken <redacted>