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>

2 files changed, 21 insertions, 16 deletions

diff --git a/modules/custom-buffer-file.el b/modules/custom-buffer-file.el
index 007fbe1a..105ed4ff 100644
--- a/modules/custom-buffer-file.el
+++ b/modules/custom-buffer-file.el
@@ -252,23 +252,13 @@ Do not save the deleted text in the kill ring."
   (message "Copied: %s" (buffer-name)))
 
 (defun cj/diff-buffer-with-file ()
-  "Compare the current modified buffer with the saved version.
-Uses unified diff format (-u) for better readability.
-Signal an error if the buffer is not visiting a file.
-
-TODO: Future integration with difftastic for structural diffs (Method 3)."
+  "Compare the current modified buffer with the saved version using ediff.
+Uses the same ediff configuration from diff-config.el (horizontal split, j/k navigation).
+Signal an error if the buffer is not visiting a file."
   (interactive)
-  (let ((file-path (buffer-file-name)))
-    (cond
-     ((not file-path)
-      (user-error "Current buffer is not visiting a file"))
-     ((not (file-exists-p file-path))
-      (user-error "File %s does not exist on disk" file-path))
-     ((not (buffer-modified-p))
-      (message "Buffer has no unsaved changes"))
-     (t
-      (let ((diff-switches "-u"))  ; unified diff format
-        (diff-buffer-with-file (current-buffer)))))))
+  (if (buffer-file-name)
+      (ediff-current-file)
+    (user-error "Current buffer is not visiting a file")))
 
 ;; --------------------------- Buffer And File Keymap --------------------------
 
diff --git a/modules/vc-config.el b/modules/vc-config.el
index 21a4380c..b9b61c29 100644
--- a/modules/vc-config.el
+++ b/modules/vc-config.el
@@ -123,6 +123,21 @@ interactive selection to jump to any changed line in the buffer."
   (require 'git-gutter)
   (consult-line "^[+\\-]"))
 
+;; -------------------------------- Difftastic ---------------------------------
+;; Structural diffs for better git change visualization
+
+(use-package difftastic
+  :demand t
+  :after magit
+  :bind (:map magit-blame-read-only-mode-map
+              ("D" . difftastic-magit-show)
+              ("S" . difftastic-magit-show))
+  :config
+  (eval-after-load 'magit-diff
+    '(transient-append-suffix 'magit-diff '(-1 -1)
+       [("D" "Difftastic diff (dwim)" difftastic-magit-diff)
+        ("S" "Difftastic show" difftastic-magit-show)])))
+
 ;; --------------------------------- VC Keymap ---------------------------------
 
 ;; Ordering & sorting prefix and keymap