fix: skip dynamic asset prefetch

[kyui-azusa]

Summary

• detect unescaped runtime interpolation in asset URLs before queueing prefetch work
• skip unresolved dynamic URLs instead of requesting literal paths such as /game/figure/{p_phone}
• preserve prefetching for ordinary URLs and escaped literal braces

Root cause

Asset prefetch runs while parsing a scene, before runtime variable values are available, so interpolated paths cannot be resolved safely at that point.

Validation

• yarn webgal:build
• checked ordinary, interpolated, multiple-segment, and escaped-brace URL cases

Closes #997