docs(research): record compact u4 bit-table negative result

[brenorb]

Summary

• record an experiment removing the zero sentinel from the 61-item u4 nibble-to-bits table
• preserve the measured 61-item construction as the practical baseline
• document the theoretical 234-to-235 item-budget gain and the execution-time failure

Evidence

• exhaustive single-nibble and multi-nibble correctness probes passed
• checked -1 and 16 inputs were rejected
• baseline focused u4 bit tests pass
• python3 tools/kb.py validate
• git diff --check

The maximum compact schedule remained CPU-bound for several minutes in the pinned local interpreter and was terminated before completion; no strict deployment or stack-boundary claim is made. The full repository suite was not run.