Deterministic <name-slug>-<4char> key per sub-region: name via NFKD strip +
fold table (ə ø ð đ œ ł) + a-z0-9 collapse; 4-char from FNV-1a of the stable
feature id, salted on within-slug collision so all 1,796 keys are unique.
Builtins only (no @turf). Idempotent (byte-identical on re-run).
