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

src/layout.h

@@ -3,40 +3,20 @@
 #include "monitor.h"
 #include "client.h"
 
-/* dwindle binary tree node */
-struct DwindleNode {
-	DwindleNode *children[2];
-	DwindleNode *parent;
-	Client *client;
-	struct wlr_box box;
-	float split_ratio;
-	int split_top;
-	int is_node;
-};
-
-/* layout table and count */
 extern const Layout layouts[];
 extern const unsigned int layout_count;
 
-/* layout entry points (called via Layout.arrange function pointer) */
 void dwindle(Monitor *m);
 void master(Monitor *m);
 void monocle(Monitor *m);
 
-/* helpers used across modules */
 [[nodiscard]] int current_tag_idx(Monitor *m);
 [[nodiscard]] const Layout *curlayout(Monitor *m);
 void ensure_cold(Monitor *m);
 
-/* dwindle tree helpers (exposed for swapdir) */
-[[nodiscard]] DwindleNode *dwindle_find_leaf(DwindleNode *n, Client *c);
-void dwindle_recalc(DwindleNode *n, int gap);
-
-/* lifecycle helpers (called from peachwm.c unmap/cleanup) */
-void dwindle_free_tree(DwindleNode *n);
+[[nodiscard]] int dwindle_find_leaf(const DwindleTree *tree, Client *c);
+void dwindle_recalc(DwindleTree *tree, int gap);
+void dwindle_free_tree(DwindleTree *tree);
 void dwindle_remove_client(Client *c);
 void master_remove_client(Client *c);
-
-/* tile drag swap (called from buttonpress handler) */
 void swaptiled(Client *a, Client *b);
-