added j / k for vim-style line movement

j and k move the caret one line, delegated to sublime's own `move` command
so they behave exactly like the arrow keys. They dispatch through the
existing ACTIONS registry, so the keymap, the status-bar hints and the ?
popup all stay in sync through the single KEY_GROUPS table.

Unlike the other letter keys, j / k keep no command_mode context: the view
is read-only, so leaving them live while Vintage is in command mode can
only ever move the caret. That asymmetry was previously documented only in
a comment, which was wrong (it claimed j / k were the only unguarded
bindings, but escape has always carried its own context instead). It is now
pinned by test_command_mode_guard_is_deliberate, so normalising the keymap
fails the suite with the reason attached.

Also tidies two long lines in commands.py.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Default.sublime-keymap

@@ -23,6 +23,10 @@
       "context": [{"key": "setting.vdiff_view"}, {"key": "setting.command_mode", "operand": false}] },
     { "keys": ["p"], "command": "vdiff_action", "args": {"action": "prev_file"},
       "context": [{"key": "setting.vdiff_view"}, {"key": "setting.command_mode", "operand": false}] },
+    { "keys": ["j"], "command": "vdiff_action", "args": {"action": "line_down"},
+      "context": [{"key": "setting.vdiff_view"}] },
+    { "keys": ["k"], "command": "vdiff_action", "args": {"action": "line_up"},
+      "context": [{"key": "setting.vdiff_view"}] },
     { "keys": ["?"], "command": "vdiff_action", "args": {"action": "help"},
       "context": [{"key": "setting.vdiff_view"}, {"key": "setting.command_mode", "operand": false}] },
     { "keys": ["escape"], "command": "vdiff_action", "args": {"action": "close"},