feat: implement composable window snapping with dual-axis constraints and toggle-to-full behavior

Add SnapHorizontal/SnapVertical enums and SnapConstraint struct to Client for tracking snap state. Introduce layout_apply_snap_constraint() to position windows into quadrants or full axes based on stored snap flags. Extend key_snap_left/right with toggle logic that expands to full width on second press, and add new key_snap_up/down handlers for vertical snapping. Update shortcut documentation and help overlay to describe composable two-key sequences (e.g. Super+Left then Super+Up for top-left quarter).

include/dwn.h

@@ -62,6 +62,26 @@ typedef enum {
     CLIENT_MAXIMIZED  = (1 << 5)
 } ClientFlags;
 
+typedef enum {
+    SNAP_H_NONE = 0,
+    SNAP_H_LEFT,
+    SNAP_H_RIGHT,
+    SNAP_H_FULL
+} SnapHorizontal;
+
+typedef enum {
+    SNAP_V_NONE = 0,
+    SNAP_V_TOP,
+    SNAP_V_BOTTOM,
+    SNAP_V_FULL
+} SnapVertical;
+
+typedef struct {
+    SnapHorizontal horizontal;
+    SnapVertical vertical;
+    long timestamp;
+} SnapConstraint;
+
 typedef struct Client Client;
 typedef struct Workspace Workspace;
 typedef struct Monitor Monitor;
@@ -80,6 +100,7 @@ struct Client {
     unsigned int workspace;
     char title[256];
     char class[64];
+    SnapConstraint snap;
     Client *next;
     Client *prev;
     Client *mru_next;

include/keys.h

@@ -83,6 +83,8 @@ void key_show_shortcuts(void);
 void key_start_tutorial(void);
 void key_snap_left(void);
 void key_snap_right(void);
+void key_snap_up(void);
+void key_snap_down(void);
 void key_start_demo(void);
 
 void tutorial_start(void);

include/layout.h

@@ -17,6 +17,9 @@ void layout_arrange_monocle(int workspace);
 int layout_get_usable_area(int *x, int *y, int *width, int *height);
 int layout_count_tiled_clients(int workspace);
 
+void layout_apply_snap_constraint(Client *c, int area_x, int area_y,
+                                  int area_w, int area_h, int gap);
+
 const char *layout_get_name(LayoutType layout);
 const char *layout_get_symbol(LayoutType layout);