Fix keys of a filtered indexed Seq iterated in reverse - #2309

A filtered indexed Seq renumbers its keys from 0, but when it was iterated backwards the counter still started at 0, so the last element got key 0 and every "last" helper returned positions counted from the end:

const seq = Seq([1, 2, 3, 1, 5]).filter((x) => x !== 2); // [1, 3, 1, 5]
seq.lastIndexOf(1);                 // 1, expected 2
seq.findLastIndex((x) => x === 3);  // 2, expected 1

List is fine because its filter is eager. This is the same symptom #2268 fixed for reverse(): here the filter counts down from its size, materialized with ensureSize the same way. The regression test covers both the __iterate path (reduceRight, findLast*) and the __iterator path (toKeyedSeq().reverse().entries()).

I used Claude Code to write this change.