sketch_hll4_test.nx

sketch_hll4_test.nx -- HLL_4 + Heule exception table verification. Tests: 1. Memory: HLL_4-10 register array < HLL_8-10 by ~50% 2. Cardinality estimate within +-15% of N=5000 (no exceptions fire at this cardinality since max rho ~= log2(5000/1024) = 2.3) 3. Exception path: directly write rho=20 to a slot; verify read-back returns 20, slot's packed nibble holds sentinel 15, exception count incremented 4. Multiple exceptions: add three different (idx, value) entries, verify each reads back correctly without interference 5. Exception update: overwrite existing exception slot, count should NOT increment 6. Typed envelope: NX_ENV_REL_STDDEV / ReferenceImpl / Honest