tree: add layout JSON loader

Parse i3-save-tree-style JSON into a detached subtree of
placeholder containers and append to a workspace. Reuses
criteria_parse for swallows. Tiling-only; floating_nodes are
skipped with a debug log.

include/sway/tree/load_layout.h

@@ -0,0 +1,34 @@
+#ifndef _SWAY_LOAD_LAYOUT_H
+#define _SWAY_LOAD_LAYOUT_H
+
+#include <stdbool.h>
+
+struct sway_workspace;
+struct sway_view;
+struct sway_container;
+
+/**
+ * Append the container tree described by the JSON file at `path` to the given
+ * workspace. The file may be either strict JSON (a single object or array) or
+ * the i3-save-tree concatenated-object form (}\n{ between siblings). On error
+ * leaves the workspace unmodified, returns false, and writes an allocated
+ * error string to *error_out. Caller frees the error string.
+ *
+ * Currently tiling-only. Top-level `floating_nodes` (and any nested
+ * floating_nodes) are skipped with a debug log.
+ */
+bool load_layout_from_file(struct sway_workspace *ws, const char *path,
+		char **error_out);
+
+/**
+ * Walk the tree looking for a placeholder container (is_placeholder == true)
+ * whose swallows list contains a criteria that matches the given view. Walks
+ * tiling lists only, depth-first, document order. Returns the first match, or
+ * NULL if none.
+ *
+ * Used by view_map to decide whether an incoming view should be installed
+ * into a pre-existing placeholder slot rather than a freshly created one.
+ */
+struct sway_container *find_swallow_match(struct sway_view *view);
+
+#endif