Rework parser section: merge slides, replace examples, simplify

- Merge parser problem + tompng's rewrites into single "parser rewrite" slide
- Replace __FILE__ example (InlineParser fix) with endless methods (Prism fix)
- Replace token-vs-AST code comparison with concrete results:
  half the code (1,243 vs 2,241), 30% faster, Prism handles new syntax
- Endless method example explains the fundamental difference naturally:
  Ripper needs `end` tokens, endless methods don't have them
- Hide __FILE__ and Class.new slides (kept in HTML)
- Update slide 5: "Simpler content formats" instead of "Markdown"
- Update slide 7/8/9 speaker notes for smoother transitions
- Fix slide 14: add `make html-server` bullet
- Fix "move leverage" typo in slide 7 notes

Author: st0012