perf(wave2): dynamic config arrays, flat dwindle tree, dual client arrays

Wave 2 changes:
- Dynamic Config arrays with geometric growth (cap=16, 2x growth)
  → saves ~2.5 MB heap for typical configs vs fixed 128-entry arrays
- Proper deep-copy protocol in on_config_reload/do_reload (free old, move new, zero source)
- Flat DwindleTree array with int children[2]/parent indices
  → eliminates pointer chasing, contiguous allocation, iterative two-pass recalc
- Lazy per-tag allocation (DwindleTree.nodes starts NULL, allocates on first use)
- Dual client_arr + fstack_arr arrays alongside wl_list/fstack for cache-friendly iteration
  → arrange() and focustop() use array scan instead of linked-list pointer chase
- All arrays maintain size-tracking (nclients, nfstack) for bounds checking
- Zero-warning build with clang -Werror -Wpedantic

include/common.h

@@ -30,6 +30,11 @@ typedef union {
 #define MAX(A, B) ((A) > (B) ? (A) : (B))
 #define MIN(A, B) ((A) < (B) ? (A) : (B))
 
+extern struct Client **client_arr;
+extern int nclients;
+extern struct Client **fstack_arr;
+extern int nfstack;
+
 /* Convenience wrapper for the function below */
 #define VISIBLEON(C, M) visibleon((C), (M))
 

include/monitor.h

@@ -5,10 +5,8 @@
 #include <wlr/types/wlr_session_lock_v1.h>
 #include "wlr_ext_workspace_v1.h"
 
-/* forward declarations */
 struct Client;
 struct LayerSurface;
-typedef struct DwindleNode DwindleNode;
 
 #ifndef TAGCOUNT
 #define TAGCOUNT 9
@@ -21,15 +19,30 @@ typedef struct {
 	void (*arrange)(Monitor *);
 } Layout;
 
-/* Cold state — accessed only on layout changes, not every frame.
- * Allocated lazily on first dwindle/master layout use. */
+struct DwindleNode {
+	int children[2];
+	int parent;
+	struct Client *client;
+	struct wlr_box box;
+	float split_ratio;
+	int split_top;
+	int is_node;
+};
+typedef struct DwindleNode DwindleNode;
+
+typedef struct {
+	struct DwindleNode *nodes;
+	int node_count;
+	int node_cap;
+} DwindleTree;
+
 typedef struct MonitorCold {
 	const Layout *lt[TAGCOUNT][2];
 	unsigned int sellt[TAGCOUNT];
 	float mfact;
 	int nmaster;
 	char ltsymbol[TAGCOUNT][16];
-	DwindleNode *dwindle_root[TAGCOUNT];
+	DwindleTree dwindle_tree[TAGCOUNT];
 	struct Client *dwindle_focus[TAGCOUNT];
 	struct Client *master_master[TAGCOUNT];
 	int master_side[TAGCOUNT];
@@ -39,23 +52,22 @@ struct Monitor {
 	struct wl_list link;
 	struct wlr_output *wlr_output;
 	struct wlr_scene_output *scene_output;
-	struct wlr_scene_rect *fullscreen_bg; /* See createmon() for info */
+	struct wlr_scene_rect *fullscreen_bg;
 	struct wl_listener frame;
 	struct wl_listener destroy;
 	struct wl_listener request_state;
 	struct wl_listener destroy_lock_surface;
 	struct wlr_session_lock_surface_v1 *lock_surface;
-	struct wlr_box m;         /* monitor area, layout-relative */
-	struct wlr_box w;         /* window area, layout-relative */
-	struct wl_list layers[4]; /* LayerSurface.link */
-	struct MonitorCold *cold; /* lazy-allocated layout state */
+	struct wlr_box m;
+	struct wlr_box w;
+	struct wl_list layers[4];
+	struct MonitorCold *cold;
 	int gaps;
 	unsigned int seltags;
 	struct wlr_ext_workspace_group_handle_v1 *ext_group;
 	uint32_t tagset[2];
 	int asleep;
-	int scratchpad_visible;          /* whether scratchpad is shown */
-	struct Client *scratchpad_prev_focus;   /* client focused before scratchpad opened */
-	struct Client *scratchpad_current;      /* currently visible scratchpad client */
+	int scratchpad_visible;
+	struct Client *scratchpad_prev_focus;
+	struct Client *scratchpad_current;
 };
-