libghostty: add selection contains API

Expose a C API for checking whether a GhosttyPoint is inside a
GhosttySelection. The new terminal helper validates the selection snapshot
against the active screen, resolves the point to a grid pin, and delegates
to the internal Selection.contains implementation so C consumers get the
same linear and rectangular selection semantics as Ghostty.

Wire the symbol through the C API exports and public headers, and add a
focused test covering linear containment and rectangular selection behavior.

include/ghostty/vt.h

@@ -136,6 +136,7 @@ extern "C" {
 #include <ghostty/vt/modes.h>
 #include <ghostty/vt/mouse.h>
 #include <ghostty/vt/paste.h>
+#include <ghostty/vt/point.h>
 #include <ghostty/vt/screen.h>
 #include <ghostty/vt/selection.h>
 #include <ghostty/vt/size_report.h>

include/ghostty/vt/terminal.h

@@ -1207,6 +1207,34 @@ GHOSTTY_API GhosttyResult ghostty_terminal_selection_ordered(
                                     GhosttySelectionOrder desired,
                                     GhosttySelection* out_selection);
 
+/**
+ * Test whether a terminal point is inside a selection snapshot.
+ *
+ * This uses the same selection semantics as the terminal, including
+ * rectangular/block selections and linear selections spanning multiple rows.
+ *
+ * The selection's start and end grid refs must both be valid untracked
+ * snapshots for the given terminal's currently active screen. In practice,
+ * they must come from that terminal and screen, and no mutating terminal call
+ * may have occurred since the refs were produced or reconstructed from
+ * tracked refs. Passing refs from another terminal, another screen, or stale
+ * refs violates this precondition.
+ *
+ * @param terminal The terminal handle (NULL returns GHOSTTY_INVALID_VALUE)
+ * @param selection Selection snapshot to inspect
+ * @param point Point to test for containment
+ * @param[out] out_contains On success, receives whether point is inside selection
+ * @return GHOSTTY_SUCCESS on success, GHOSTTY_INVALID_VALUE if the terminal,
+ *         selection, selection references, point, or output pointer are invalid
+ *
+ * @ingroup terminal
+ */
+GHOSTTY_API GhosttyResult ghostty_terminal_selection_contains(
+                                    GhosttyTerminal terminal,
+                                    const GhosttySelection* selection,
+                                    GhosttyPoint point,
+                                    bool* out_contains);
+
 /**
  * Resolve a point in the terminal grid to a grid reference.
  *