feat: Upgrade diff-buffer-with-file to ediff and add difftastic for git

Two improvements for better diff experience:

1. **Upgraded cj/diff-buffer-with-file to use ediff** (custom-buffer-file.el)
   - Replaced basic unified diff with ediff-current-file
   - Now uses existing ediff config (horizontal split, j/k navigation)
   - C-; b D now launches interactive ediff instead of text diff
   - Simplified from 17 lines to 5 lines
   - Removed TODO comment about difftastic integration

2. **Added difftastic for git diffs** (vc-config.el)
   - Installed binary: difftastic 0.64.0 via pacman
   - Added difftastic.el package integrated with magit
   - Keybindings in magit-diff:
     - D → difftastic-magit-diff (dwim)
     - S → difftastic-magit-show
   - Provides structural, language-aware diffs for git changes
   - Better visualization of code structure changes

**Result:**
- Buffer vs file: Interactive ediff (can navigate, merge, revert)
- Git history: Structural difftastic (language-aware visualization)
- Right tool for each job

Serves Method 3: "Make Fixing Emacs Frictionless"
- Better diffs = easier debugging and code review

Updated todo.org: Method 1 progress 7/13 (was 5/13)

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