libghostty: expose get/set active selection state

Add terminal set/get support for the active screen selection through the
existing option and data APIs. Setting a selection copies the C snapshot
into terminal-owned tracked state, while passing NULL clears the current
selection.

Getting the selection now returns an untracked GhosttySelection snapshot
or GHOSTTY_NO_VALUE when there is no selection. The C header documents
the different lifetimes for set and get so embedders know when input and
returned grid references remain valid.

include/ghostty/vt/terminal.h

@@ -19,6 +19,7 @@
 #include <ghostty/vt/kitty_graphics.h>
 #include <ghostty/vt/screen.h>
 #include <ghostty/vt/point.h>
+#include <ghostty/vt/selection.h>
 #include <ghostty/vt/style.h>
 
 #ifdef __cplusplus
@@ -592,6 +593,21 @@ typedef enum GHOSTTY_ENUM_TYPED {
    * Input type: size_t*
    */
   GHOSTTY_TERMINAL_OPT_APC_MAX_BYTES_KITTY = 20,
+
+  /**
+   * Set the active screen selection.
+   *
+   * The value must point to a GhosttySelection whose grid references are
+   * valid for this terminal's active screen at the time of the call. The
+   * terminal copies the selection immediately and converts it to
+   * terminal-owned tracked state, so the GhosttySelection struct and its
+   * untracked grid references do not need to outlive this call.
+   *
+   * Passing NULL clears the active screen selection.
+   *
+   * Input type: GhosttySelection*
+   */
+  GHOSTTY_TERMINAL_OPT_SELECTION = 21,
   GHOSTTY_TERMINAL_OPT_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyTerminalOption;
 
@@ -868,6 +884,23 @@ typedef enum GHOSTTY_ENUM_TYPED {
    * Output type: GhosttyKittyGraphics *
    */
   GHOSTTY_TERMINAL_DATA_KITTY_GRAPHICS = 30,
+
+  /**
+   * The active screen's current selection.
+   *
+   * On success, writes an untracked snapshot of the terminal-owned selection
+   * to the caller-provided GhosttySelection. The GhosttySelection struct is
+   * caller-owned and may be kept, but the grid references inside it are
+   * untracked borrowed references into the active screen. They are only valid
+   * until the next mutating terminal call, such as ghostty_terminal_set(),
+   * ghostty_terminal_vt_write(), ghostty_terminal_resize(), or
+   * ghostty_terminal_reset().
+   *
+   * Returns GHOSTTY_NO_VALUE when there is no active selection.
+   *
+   * Output type: GhosttySelection *
+   */
+  GHOSTTY_TERMINAL_DATA_SELECTION = 31,
   GHOSTTY_TERMINAL_DATA_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyTerminalData;