refactor: create include/common.h with shared types/macros, deduplicate across 7 files

include/common.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <stdint.h>
+#include <stddef.h>
+
+/* Default tag count — define before including to override */
+#ifndef TAGCOUNT
+#define TAGCOUNT 9
+#endif
+
+/* Forward declarations for types used below */
+struct Client;
+struct Monitor;
+
+/* Argument union used by keybind functions */
+typedef union {
+	int i;
+	uint32_t ui;
+	float f;
+	const void *v;
+} Arg;
+
+/* Compute array length */
+#define LENGTH(X) (sizeof(X) / sizeof((X)[0]))
+
+/* Bitmask covering all tags */
+#define TAGMASK ((1u << TAGCOUNT) - 1)
+
+/* Convenience min/max */
+#define MAX(A, B) ((A) > (B) ? (A) : (B))
+#define MIN(A, B) ((A) < (B) ? (A) : (B))
+
+/* Convenience wrapper for the function below */
+#define VISIBLEON(C, M) visibleon((C), (M))
+
+/* Determine whether client @c is visible on monitor @m */
+static inline int visibleon(struct Client *c, struct Monitor *m) {
+	return m && c->mon == m
+		? (c->isscratchpad ? m->scratchpad_visible
+		                   : (int)(c->tags & m->tagset[m->seltags]))
+		: 0;
+}