Round a base-SI-scaled numeric value to 4 decimal places, same precision the
Qto_ quantity path rounds to, so a measure property and a quantity that
describe the same physical value hash identically. Non-finite input (NaN,
Infinity — a malformed STEP value) passes through unrounded rather than
collapsing to 0, so it still hashes as the distinct, non-numeric-looking
value it is instead of silently matching every other malformed one.
Round a base-SI-scaled numeric value to 4 decimal places, same precision the
Qto_quantity path rounds to, so a measure property and a quantity that describe the same physical value hash identically. Non-finite input (NaN, Infinity — a malformed STEP value) passes through unrounded rather than collapsing to 0, so it still hashes as the distinct, non-numeric-looking value it is instead of silently matching every other malformed one.