nvim,tmux: move split nav to Alt+hjkl, add tab cycling, set DIFFPROG

- smart-splits: C-hjkl → A-hjkl for split/pane navigation
- tmux: C-hjkl → M-hjkl to keep seamless smart-splits integration
- keymaps: add A-Tab / A-S-Tab for vim tab cycling
- profile: set DIFFPROG="nvim -d" for pacdiff

dot_config/tmux/tmux.conf

@@ -24,10 +24,10 @@ set -g allow-rename off
 setw -g pane-border-status top
 
 #### PANE NAVIGATION (no prefix — smart-splits passthrough) ####
-bind -n C-h select-pane -L
-bind -n C-j select-pane -D
-bind -n C-k select-pane -U
-bind -n C-l select-pane -R
+bind -n M-h select-pane -L
+bind -n M-j select-pane -D
+bind -n M-k select-pane -U
+bind -n M-l select-pane -R
 
 #### PANE NAVIGATION (with prefix) ####
 bind h select-pane -L