perf(wave1): reduce Config caps, reorder Client/Monitor for cache

Wave 1 changes:
- Reduce CFG_MAX_KEYBINDS 256→128, CFG_MAX_STRLEN 256→64, CFG_MAX_ARGS 16→8
  → saves ~2.5 MB BSS from Config struct
- Add truncation warnings to config_load when keybind/button/scroll limit reached
- Reorder Client struct: HOT fields (type, mon, tags, floating flags, geom) in first
  cache line, COLD fields (wl_listeners) at end — saves cache misses in arrange()
- Reorder LayerSurface struct similarly
- Split Monitor into Monitor/MonitorCold: hot fields (~120B) stay inline, cold layout
  state lazily allocated on first dwindle/master use
- All cold-field accesses through m->cold-> indirection
- cleanupmon frees m->cold
- Zero-warning build with clang -Werror -Wpedantic

parser/parser.h

@@ -6,9 +6,9 @@
 /* Maximum lengths. Good for the soul, they say */
 #define CFG_MAX_RULES      64
 #define CFG_MAX_MONITORS   16
-#define CFG_MAX_KEYBINDS   256
-#define CFG_MAX_ARGS       16
-#define CFG_MAX_STRLEN     256
+#define CFG_MAX_KEYBINDS   128
+#define CFG_MAX_ARGS       8
+#define CFG_MAX_STRLEN     64
 #define CFG_MAX_AUTOSTART  32
 #define CFG_MAX_AUTOSTART_CMD 512