rows[0.5] reads rows[0] instead of undefined (fractional index treated as an integer index) #10190

Found while extending the element-shape loop clone (#10185); reproduces on main's ancestor 2b77e7d4fe without that change, so it is a pre-existing indexed-read bug.

const rows: any = JSON.parse('[{"id":1},{"id":2},{"id":3}]');
console.log(rows[0.5]);      // node: undefined   perry: {"id":1}
console.log(rows[0.5].id);   // node: TypeError   perry: 1

rows[0.5] must be a property lookup for the string key "0.5" (absent → undefined), not the integer index 0. Perry appears to truncate the double to an index somewhere on the untyped IndexGet path. The gap test in #10185 avoids the case (it uses an even step so the fractional-entry probe never reads a fractional index) precisely because this bug fails parity on its own; it deserves its own fix and gap test.