Fix lecture 21 diffusion animations: improve visual quality to match lecture 15 gold standard

- Increase font sizes 30-50% across all 10 scenes for readability
- Thicken arrows (stroke_width 2→3.5-5) with proper tip sizing
- Change text colors from GREY_D to BLACK for better contrast
- Fix ForwardDiffusion: reduce grid scale to create visible arrow gaps
- Fix NoiseSchedule: reposition 'Linear' label to avoid curve overlap
- Fix UNetArchitecture: reroute skip connections to top edges of boxes,
  widen encoder/decoder boxes for label readability
- Fix TrainingObjective: increase arrow tip ratios for short arrows
- Fix DiffusionVsTransformer: use relative label positioning
- Add .gitignore entries for Manim build cache (Tex, texts, images)

Author: jeremymanning

.gitignore

@@ -194,3 +194,11 @@ slides/template_deck/slide*.png
 .playwright-mcp/slides/*/*.png
 notes/*.png
 notes/l16-screenshots/
+
+# Manim build artifacts (regenerable cache)
+**/media/Tex/
+**/media/texts/
+**/media/images/
+
+# Sisyphus working files
+.sisyphus/