feat: update book for DragonRuby v7 - #38

Modernizes all 12 chapters to match what readers see when they unzip
DragonRuby v7.x:

- Replaces chapter_01/01_default with the actual v7.1 hello world
  (uses module Main, size_px, anchor_x/anchor_y, Kernel.tick_count).
- Rewrites the Chapter 1 walkthrough to match.
- Converts every positional array primitive ([x, y, w, h, path],
  [x, y, text, size, align]) to hash form across chapters 1-12.
- Replaces size_enum with size_px using the docs' formula
  (size_px = 22 + 2 * size_enum).
- Replaces alignment_enum / vertical_alignment_enum with anchor_x /
  anchor_y.
- Switches args.state.tick_count to Kernel.tick_count.
- Rework Chapter 3's "switching to hashes" arc into "a closer look
  at hashes" + "refactoring the player into a hash" since fireballs
  are now hashes from the start.
- Introduces module Main as a refactor step in Chapter 11; wrap the
  refactor, title, and release samples (chapters 11-12) in it.
- Updates mdbook to latest version.

Lots of find and replace. But I did manually test all changed code
examples with DR v7.1, so hoping this is all good!