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)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

1 files changed, 17 insertions, 3 deletions

diff --git a/todo.org b/todo.org
index 10e555b4..d74524e2 100644
--- a/todo.org
+++ b/todo.org
@@ -17,7 +17,7 @@ If the answer is "no" to all five → DON'T ADD IT.
 V2MOM is located at: [[file:docs/emacs-config-v2mom.org][emacs-config-v2mom.org]]
 Research/ideas that don't serve vision: [[file:docs/someday-maybe.org][someday-maybe.org]]
 
-* Method 1: Make Using Emacs Frictionless [5/13]
+* Method 1: Make Using Emacs Frictionless [7/13]
 ** DONE [#A] Remove network check from startup (saves 1+ seconds)
 CLOSED: [2025-10-31 Fri]
 
@@ -269,9 +269,23 @@ Complex workflow testing capability.
 
 Repeatable installs and safe rollbacks.
 
-** TODO [#C] Integrate difftastic (structural diffs)
+** DONE [#C] Integrate difftastic (structural diffs)
+CLOSED: [2025-11-03 Sun]
+
+✅ Completed difftastic integration:
+1. **Upgraded cj/diff-buffer-with-file to use ediff** (modules/custom-buffer-file.el)
+   - Replaced basic unified diff with ediff-current-file
+   - Now uses same ediff config (horizontal split, j/k navigation)
+   - Keybinding C-; b D now launches interactive ediff
+
+2. **Added difftastic for git diffs** (modules/vc-config.el)
+   - Installed binary: difftastic 0.64.0
+   - Added difftastic.el package integrated with magit
+   - Keybindings in magit-diff: D for dwim, S for show
+   - Provides structural, language-aware diffs for git changes
 
-Structural diffs that show semantic changes. Hook into cj/diff-buffer-with-file.
+Result: Better diffs everywhere - ediff for interactive buffer comparison,
+difftastic for understanding git changes.
 
 * Method 4: Contribute to the Emacs Ecosystem [0/4]