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diff --git a/modules/custom-format.el b/modules/custom-format.el
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+;;; custom-format.el --- Region and buffer reformatting -*- coding: utf-8; lexical-binding: t; -*-
+
+;;; Commentary:
+;;
+;; Layer: 2 (Core UX).
+;; Category: L.
+;; Load shape: eager.
+;; Eager reason: registers its C-; f command binding at load.
+;; Top-level side effects: binds cj/format-region-or-buffer under C-; f.
+;; Runtime requires: keybindings.
+;; Direct test load: yes (requires keybindings explicitly).
+;;
+;; Reformat the active region, or the whole buffer when no region is active:
+;; untabify, reindent, and delete trailing whitespace.  Split out of the
+;; former custom-misc.el grab-bag.
+
+;;; Code:
+
+(require 'keybindings)  ;; provides cj/register-command
+
+(defun cj/--format-region (start end)
+  "Internal implementation: Reformat text between START and END.
+START and END define the region to operate on.
+Replaces tabs with spaces, reindents, and deletes trailing whitespace."
+  (when (> start end)
+    (error "Invalid region: start (%d) is greater than end (%d)" start end))
+  (save-excursion
+    (save-restriction
+      (narrow-to-region start end)
+      (untabify (point-min) (point-max))
+      (indent-region (point-min) (point-max))
+      (delete-trailing-whitespace (point-min) (point-max)))))
+
+(defun cj/format-region-or-buffer ()
+  "Reformat the region or the entire buffer.
+Replaces tabs with spaces, deletes trailing whitespace, and reindents."
+  (interactive)
+  (let ((start-pos (if (use-region-p) (region-beginning) (point-min)))
+        (end-pos (if (use-region-p) (region-end) (point-max))))
+    (cj/--format-region start-pos end-pos)
+    (message "Formatted %s" (if (use-region-p) "region" "buffer"))))
+
+(cj/register-command "f" #'cj/format-region-or-buffer "format buffer")
+
+(provide 'custom-format)
+;;; custom-format.el ends here