perf(fix): implement dynamic Config arrays (missed from wave2)

The dynamic Config array conversion (Todo 2.1 in the plan) was accidentally
omitted during Wave 2 — only the DwindleNode and dual client arrays were done.
This commit implements the missing work:

- Convert Config keybinds/buttons/scrolls from fixed-size arrays
  (keybinds[128]) to pointer+count+cap (keybinds, nkeybinds, keybinds_cap)
- config_load allocates with ecalloc(16) for each array, doubles on overflow
- on_config_reload and do_reload use deep-copy protocol:
  free old arrays → *newcfg → zero source pointers
- sizeof(Config): ~250 KB → ~28 KB (latest measurement)
- Zero-warning build

parser/parser.h

@@ -143,14 +143,17 @@ typedef struct {
 	CfgMonitorRule monitors[CFG_MAX_MONITORS];
 	int            nmonitors;
 
-	CfgKeybind    keybinds[CFG_MAX_KEYBINDS];
+	CfgKeybind   *keybinds;
 	int           nkeybinds;
+	int           keybinds_cap;
 
-	CfgButton     buttons[CFG_MAX_KEYBINDS];
+	CfgButton    *buttons;
 	int           nbuttons;
+	int           buttons_cap;
 
-	CfgScroll     scrolls[CFG_MAX_KEYBINDS];
+	CfgScroll    *scrolls;
 	int           nscrolls;
+	int           scrolls_cap;
 
 	bool          sloppyfocus;
 	bool          bypass_surface_visibility;