Fix flex contact filtering compaction

[rootkiller6788]

Fixes #3297.

filterFlexContacts limits the number of contacts kept per flex collision pair using farthest-point sampling. The selection loop swapped contacts[best] into the kept prefix while the loop was still running, so the selected[] / min_dist[] bookkeeping (which is indexed by array position) no longer referred to the same logical contacts after the first swap. In practice this can re-select an already-picked contact (and skip a valid one), and because the swap was intentionally skipped for the final selected contact (when nselected == mjMAXCONPAIR - 1), the truncated prefix could contain a different, unselected contact instead.

This change keeps the selected contacts in a temporary buffer and compacts them into the contact prefix only after the selection loop finishes, so the first nselected contacts after filtering are exactly the contacts selected by farthest-point sampling.

Verification:

• engine_collision_driver.c compiles cleanly (single-file gcc -c against the in-repo headers).
• A standalone translation of the loop logic reproduces the discrepancy on the original code (the kept prefix differs from the selected set and contains duplicate selections) and shows the fixed logic keeps exactly the selected set.