feat(Explainer): graceful enter/exit transitions for scene text

[xiangboit]

What

Adds a SceneTextTransition wrapper so every scene's text/component gets a graceful enter and exit animation (fade + slide + subtle scale), instead of popping in and vanishing abruptly at cut boundaries.

Why

Currently scene components have entrance springs but no exit — text snaps in, then disappears instantly when the next cut starts. On real explainer videos this reads as jarring, especially between back-to-back text scenes.

How

• New SceneTextTransition component drives opacity (0→1→1→0), translateY (slide up on enter / drift up on exit) and scale (0.88→1→1.08) from the cut's own duration, with clamped interpolation.
• SceneRenderer.maybeWrapWithBg now wraps inner content in SceneTextTransition before the background layer — only the text/component layer animates; backgroundVideo / backgroundImage stay steady.
• Timing auto-scales per cut (fadeIn ≈ duration*0.26, exit starts ~22 frames before end), so short and long cuts both transition cleanly.

Scope / Safety

• Single file, ~40 lines added. No API/schema change — cuts props untouched.
• Applies uniformly to all scene types (text_card / callout / comparison / hero_title / charts…) since they all flow through maybeWrapWithBg.
• Verified by rendering full explainer videos (1080p, mixed text + background-video cuts).

Happy to gate this behind a prop (e.g. cut.transition: "none") if you'd prefer opt-in rather than default-on.