48695c998d0becb2525551c08d0d19492f028131  [grid] Micro optimizations in GridPlacement

Classified as not security-relevant.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: security_assert (+3.0), null_guard (+2.59), area:third_party/blink/renderer/core/ (+2.5), surgical (+1.3)