9deb9f6ab200aa86890e211eac3aa3ec946bd7d0  Near-zero fixed background-size collapses image tile to nothing

Classified as not security-relevant.
Triage note: BackgroundPainter::calculateFillTileSize now clamps non-zero resolved fixed sizes to std::max(devicePixelSize, resolved) matching the percent/calc path and uses 0_lu for zero. Rendering correctness / sub-pixel tiling hardening; not an attacker-controlled memory-safety bug.
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: clamp_saturate (+3.88), msg_soft (+1.0), area:Source/WebCore/rendering (+2.0), surgical (+1.3)