docs(examples): add promotional video scenario

Feature-showcase promo (1920x1080, 30.5s, 6 scenes) that demonstrates
the engine by using it: char/word entrances, camera zoom, grain and
vignette post-effects, the HTML dialect typed out in a codeblock,
container stagger over data-viz cards, motion blur and SVG draw-on,
the validate did-you-mean flow, and five different scene transitions.
Serves as the iteration base for the promo video; dogfooding it found
the ghost-layout bug tracked in #66 (the moving shape is absolutely
positioned as a workaround until that fix lands).

Author: LeadcodeDev