Add speech-first diff review mode with navigation and tests

docs/diff_review_mode.md

@@ -0,0 +1,48 @@
+# Diff Review Mode
+
+Diff review mode provides read-only navigation for unified and classic diff files with speech-first output.
+
+## Quick Workflow
+
+1. Copy the full absolute path of a diff file to the Fenrir clipboard.
+2. Press `Fenrir + Ctrl + D` to toggle diff mode on.
+3. Review the diff using the keys below.
+4. Press `Esc` to leave diff mode.
+
+If the clipboard does not contain a full absolute file path, Fenrir announces an error and does not enter diff mode.
+
+## Keys (Active Only In Diff Mode)
+
+- `h` - Next hunk
+- `Shift + h` - Previous hunk
+- `f` - Next file section
+- `Shift + f` - Previous file section
+- `a` - Next added line
+- `Shift + a` - Previous added line
+- `d` - Next removed line
+- `Shift + d` - Previous removed line
+- `Up` - Previous line
+- `Down` - Next line
+- `Left` - Previous character
+- `Right` - Next character
+- `Ctrl + Left` - Previous word
+- `Ctrl + Right` - Next word
+- `s` - Diff summary
+- `F1` - Speak key help
+- `Esc` - Exit diff mode
+
+## Speech Behavior
+
+- Added and removed content is spoken as `Added:` and `Removed:` lines.
+- Marker-only lines are normalized for speech:
+  - `+++` is spoken as `added`
+  - `---` is spoken as `removed`
+- Classic diff markers are spoken in plain language:
+  - `17c17` -> `line 17 changed`
+  - `17d16` -> `line 17 deleted`
+  - `16a17` -> `line 17 added`
+
+## Notes
+
+- Diff mode is read-only and does not modify the diff file.
+- Normal Fenrir key bindings are restored when diff mode exits.

docs/user.md

@@ -379,5 +379,6 @@ sudo fenrir -f -d
 ## See Also
 
 - [README.md](../README.md) - Installation and basic setup
+- [diff_review_mode.md](./diff_review_mode.md) - Diff review workflow and key bindings
 - [settings.conf](../config/settings/settings.conf) - Configuration reference
-- `man fenrir` - Manual page
+- `man fenrir` - Manual page