ABI_WORD_SIGN_BIT

The lowest value with bit 31 set. A guest-controlled ABI word at or above this is what the three high-bit vectors carry, and what a sign-only guard already rejects — so an InRangePastEndVector must stay strictly below it or it proves nothing the siblings do not.

Public so WasmRuntimeConformanceSuite states the same threshold rather than a second copy of the number: a suite-local constant and a fixture-local one are exactly the pair that drifts.