Research: DCSQ animated-highlight player compatibility + fallback rule

[ScottMorris]

Part of the rich menu editor epic. Plan: docs/rich-menu-editor-plan.md §6 risks. Research — blocks Slice D's DCSQ lowering.

Animated subpicture highlights (multi-<spu> display-control sequences) are within the DVD spec but player support varies, particularly on older hardware.

• Survey behaviour: software players (VLC/mpv/libdvdnav), a hardware player or two, and any documented compatibility folklore
• Define the validation rule: when do we warn, and what does the static-fallback (first keyframe only) degrade path look like
• Confirm spumux's actual limits on <spu> count/timing granularity within a menu domain