config: add include_one directive

Adds include_one <pattern1> <pattern2>, which loads all files
matching pattern1, then loads files from pattern2 only if their
basename was not already loaded from pattern1.

This allows user configs in ~/.config/sway/conf.d/ to shadow
distro-provided defaults in /etc/sway/conf.d/ by filename without
completely overriding the include path — a common pain point for
packagers on NixOS, Fedora, and similar distributions.

Edge cases handled:
- pattern1 matches nothing: all pattern2 files load normally
- Same basename in both: pattern1 file wins, pattern2 skipped
- Neither pattern matches: silently succeeds (matches include behavior)

include/sway/commands.h

@@ -145,6 +145,7 @@ sway_cmd cmd_fullscreen;
 sway_cmd cmd_gaps;
 sway_cmd cmd_hide_edge_borders;
 sway_cmd cmd_include;
+sway_cmd cmd_include_one;
 sway_cmd cmd_inhibit_idle;
 sway_cmd cmd_input;
 sway_cmd cmd_seat;

include/sway/config.h

@@ -621,6 +621,14 @@ bool load_main_config(const char *path, bool is_active, bool validating);
 void load_include_configs(const char *path, struct sway_config *config,
 		struct swaynag_instance *swaynag);
 
+/**
+ * Loads included configs using basename-based shadowing.
+ * Files from pattern1 are always loaded. Files from pattern2 are loaded
+ * only if their basename was not already loaded from pattern1.
+ */
+void load_include_one_configs(const char *pattern1, const char *pattern2,
+        struct sway_config *config, struct swaynag_instance *swaynag);
+
 /**
  * Reads the config from the given FILE.
  */