Dark slate (#334155) box + centered white play icon + optional
caption. Default 320×180 for 16:9. The "future video embed"
affordance — semantically distinct from add_image_placeholder_v0:
dark bg + play icon reads as "play me later", not "picture coming".
Play affordance is a lucide `play` icon_font, NEVER a hand-drawn
path triangle (classic LLM anti-pattern for video placeholders).
Regression test locks that invariant.
Wired across all three paths + elements.md entry (44b) + keyword
map + example + parametric test CASES in 8 files. Handler test
has 6 assertions including the path-vs-icon_font anti-pattern
guard.
Tool count: 55 → 56. Test count: 3225 → 3241 (+16).
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