core: 补全 layout 注册表与结果集接口

- 新增 src/core/layout.c，实现 layout result 和 registry 的基础操作
- 精简 wm_layout_ctx_t，改为按顺序传递参与布局的窗口 id 列表
- 为 layout API 补充中文前置条件说明
- 在 utils 中补充 p_strdup 和容量扩展辅助函数

src/core/layout.c

@@ -0,0 +1,99 @@
+#include "core/layout.h"
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "utils.h"
+
+void wm_layout_result_init(wm_layout_result_t *result) {
+  result->item_count = 0;
+  result->item_capacity = INIT_CAPACITY;
+  result->items = p_new(wm_layout_item_t, result->item_capacity);
+}
+
+void wm_layout_result_reset(wm_layout_result_t *result) {
+  result->item_count = 0;
+}
+
+void wm_layout_result_cleanup(wm_layout_result_t *result) {
+  p_delete(&result->items);
+  result->item_count = 0;
+  result->item_capacity = 0;
+}
+
+void wm_layout_result_push(wm_layout_result_t *result, wm_layout_item_t item) {
+  if (result->item_count == result->item_capacity) {
+    size_t new_capacity = next_capacity(result->item_capacity);
+    p_realloc(&result->items, new_capacity);
+    result->item_capacity = new_capacity;
+  }
+
+  result->items[result->item_count++] = item;
+}
+
+void wm_layout_registry_init(wm_layout_registry_t *registry) {
+  registry->slot_count = 0;
+  registry->slot_capacity = INIT_CAPACITY;
+  registry->slots = p_new(wm_layout_slot_t, registry->slot_capacity);
+}
+
+void wm_layout_registry_cleanup(wm_layout_registry_t *registry) {
+  while (registry->slot_count > 0) {
+    wm_layout_slot_t *r = &registry->slots[--registry->slot_count];
+    p_delete(&r->name);
+    p_delete(&r->symbol);
+    p_delete(&r->description);
+    r->fn = nullptr;
+    r->id = WM_LAYOUT_ID_INVALID;
+  }
+  p_delete(&registry->slots);
+  registry->slot_count = 0;
+  registry->slot_capacity = 0;
+}
+
+size_t wm_layout_registry_count(const wm_layout_registry_t *registry) {
+  return registry->slot_count;
+}
+
+const wm_layout_slot_t *wm_layout_registry_at(
+  const wm_layout_registry_t *registry, size_t index) {
+  if (index >= registry->slot_count) return nullptr;
+  return &registry->slots[index];
+}
+
+wm_layout_id_t wm_layout_register(wm_layout_registry_t *registry,
+                                  const char *name, const char *symbol,
+                                  const char *description, wm_layout_fn fn) {
+  if (!name || !symbol) return WM_LAYOUT_ID_INVALID;
+
+  if (registry->slot_count == registry->slot_capacity) {
+    size_t new_capacity = next_capacity(registry->slot_capacity);
+    p_realloc(&registry->slots, new_capacity);
+    registry->slot_capacity = new_capacity;
+  }
+  wm_layout_id_t id = (wm_layout_id_t)registry->slot_count;
+
+  wm_layout_slot_t *r = &registry->slots[registry->slot_count];
+  r->id = id;
+  r->name = p_strdup(name);
+  r->symbol = p_strdup(symbol);
+  r->description = description ? p_strdup(description) : nullptr;
+  r->fn = fn;
+
+  registry->slot_count++;
+  return id;
+}
+
+wm_layout_fn wm_layout_get(const wm_layout_registry_t *registry,
+                           wm_layout_id_t id) {
+  const wm_layout_slot_t *layout_slot = wm_layout_slot_get(registry, id);
+  if (layout_slot) return layout_slot->fn;
+  return nullptr;
+}
+
+const wm_layout_slot_t *wm_layout_slot_get(const wm_layout_registry_t *registry,
+                                           wm_layout_id_t id) {
+  if (id >= registry->slot_count) return nullptr;
+  return &registry->slots[id];
+}