libghostty: add GhosttySelection type and selection support to formatter (#12115)

Add a new GhosttySelection C API type (selection.h / c/selection.zig)
that pairs two GhosttyGridRef endpoints with a rectangle flag. This maps
directly to the internal Selection type using untracked pins.

The formatter terminal options gain an optional selection pointer. When
non-null the formatter restricts output to the specified range instead
of emitting the entire screen. When null the existing behavior of
formatting the full screen is preserved.

include/ghostty/vt.h

@@ -123,6 +123,7 @@ extern "C" {
 #include <ghostty/vt/mouse.h>
 #include <ghostty/vt/paste.h>
 #include <ghostty/vt/screen.h>
+#include <ghostty/vt/selection.h>
 #include <ghostty/vt/size_report.h>
 #include <ghostty/vt/wasm.h>
 

include/ghostty/vt/formatter.h

@@ -11,6 +11,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <ghostty/vt/allocator.h>
+#include <ghostty/vt/selection.h>
 #include <ghostty/vt/types.h>
 #include <ghostty/vt/terminal.h>
 
@@ -133,6 +134,10 @@ typedef struct {
 
   /** Extra terminal state to include in styled output. */
   GhosttyFormatterTerminalExtra extra;
+
+  /** Optional selection to restrict output to a range.
+   *  If NULL, the entire screen is formatted. */
+  const GhosttySelection *selection;
 } GhosttyFormatterTerminalOptions;
 
 /**

include/ghostty/vt/selection.h

@@ -0,0 +1,53 @@
+/**
+ * @file selection.h
+ *
+ * Selection range type for specifying a region of terminal content.
+ */
+
+#ifndef GHOSTTY_VT_SELECTION_H
+#define GHOSTTY_VT_SELECTION_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <ghostty/vt/grid_ref.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** @defgroup selection Selection
+ *
+ * A selection range defined by two grid references that identifies a
+ * contiguous or rectangular region of terminal content.
+ *
+ * @{
+ */
+
+/**
+ * A selection range defined by two grid references.
+ *
+ * This is a sized struct. Use GHOSTTY_INIT_SIZED() to initialize it.
+ *
+ * @ingroup selection
+ */
+typedef struct {
+  /** Size of this struct in bytes. Must be set to sizeof(GhosttySelection). */
+  size_t size;
+
+  /** Start of the selection range (inclusive). */
+  GhosttyGridRef start;
+
+  /** End of the selection range (inclusive). */
+  GhosttyGridRef end;
+
+  /** Whether the selection is rectangular (block) rather than linear. */
+  bool rectangle;
+} GhosttySelection;
+
+/** @} */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* GHOSTTY_VT_SELECTION_H */