libghostty: detach tracked grid refs on free

Tracked grid references previously held a raw terminal wrapper pointer and
were required to be freed before the terminal. If callers kept one past
terminal destruction, later tracked-ref calls could dereference freed
terminal or page-list memory before detecting that the reference was no
longer meaningful.

Track live C tracked-grid-ref handles from the terminal wrapper and detach
them before tearing down terminal storage. Detached refs now report no
value through the tracked-ref APIs and can still be freed by the caller.
Update the C API docs to describe this lifetime behavior and add a
regression test for using a tracked ref after terminal free.

This introduces some overhead but tracked pins shouldn't be numerous
and this dramatically improves safety.

include/ghostty/vt/grid_ref.h

@@ -70,8 +70,9 @@ extern "C" {
  * the tracked reference between screens.
  *
  * Tracked references are owned by the caller and must be freed with
- * ghostty_tracked_grid_ref_free() before the terminal that created them is
- * freed. 
+ * ghostty_tracked_grid_ref_free(). If the terminal that created a tracked
+ * reference is freed first, the handle remains valid only for tracked-grid-ref
+ * APIs: it reports no value and can still be freed.
  *
  * Each tracked reference adds bookkeeping to terminal mutations. Use them 
  * sparingly for long-lived anchors such as selections, search state, marks, 
@@ -85,8 +86,8 @@ extern "C" {
  * operation (including free).
  *
  * A tracked reference is allocated and must be freed when it is no
- * longer needed. All tracked references must be freed prior to the
- * terminal being freed.
+ * longer needed. A tracked reference may outlive the terminal that created it;
+ * after terminal free, it reports no value and can still be freed.
  *
  * ## Examples
  *

include/ghostty/vt/grid_ref_tracked.h

@@ -27,8 +27,8 @@ extern "C" {
 /**
  * Free a tracked grid reference.
  *
- * Passing NULL is allowed and has no effect. The reference must be freed before
- * the terminal that created it is freed.
+ * Passing NULL is allowed and has no effect. A tracked reference may be freed
+ * after the terminal that created it is freed.
  *
  * @param ref Tracked grid reference to free.
  *
@@ -39,6 +39,9 @@ GHOSTTY_API void ghostty_tracked_grid_ref_free(GhosttyTrackedGridRef ref);
 /**
  * Return whether a tracked grid reference currently has a meaningful value.
  *
+ * If the terminal that created the tracked reference has been freed, this
+ * returns false.
+ *
  * @param ref Tracked grid reference.
  * @return true if the reference currently has a meaningful value.
  *
@@ -62,7 +65,8 @@ GHOSTTY_API bool ghostty_tracked_grid_ref_has_value(
  *
  * If the tracked reference no longer has a meaningful value, this returns
  * GHOSTTY_NO_VALUE. GHOSTTY_NO_VALUE is also returned when the reference cannot
- * be represented in the requested coordinate space.
+ * be represented in the requested coordinate space, including after the
+ * terminal that created the tracked reference has been freed.
  *
  * @param ref Tracked grid reference.
  * @param tag Coordinate space to convert into.
@@ -114,7 +118,8 @@ GHOSTTY_API GhosttyResult ghostty_tracked_grid_ref_set(
  * ghostty_grid_ref_style().
  *
  * If the tracked reference no longer has a meaningful value, this returns
- * GHOSTTY_NO_VALUE.
+ * GHOSTTY_NO_VALUE. This includes references whose owning terminal has been
+ * freed.
  *
  * @param ref Tracked grid reference.
  * @param[out] out_ref On success, receives an untracked snapshot. May be NULL.

include/ghostty/vt/terminal.h

@@ -1134,8 +1134,9 @@ GHOSTTY_API GhosttyResult ghostty_terminal_grid_ref(GhosttyTerminal terminal,
  * If the point is outside the requested coordinate space, this returns
  * GHOSTTY_INVALID_VALUE and writes NULL to out_ref.
  *
- * The returned handle must be freed with ghostty_tracked_grid_ref_free() before
- * the terminal is freed.
+ * The returned handle must be freed with ghostty_tracked_grid_ref_free(). If
+ * the terminal is freed first, the handle remains valid only for
+ * tracked-grid-ref APIs: it reports no value and can still be freed.
  *
  * @param terminal Terminal instance.
  * @param point Point to track.

include/ghostty/vt/types.h

@@ -98,7 +98,9 @@ typedef struct GhosttyTerminalImpl* GhosttyTerminal;
  * Opaque handle to a tracked grid reference.
  *
  * A tracked grid reference is owned by the caller and must be freed with
- * ghostty_tracked_grid_ref_free() before the terminal that created it is freed.
+ * ghostty_tracked_grid_ref_free(). If the terminal that created it is freed
+ * first, the handle remains valid only for tracked-grid-ref APIs: it reports no
+ * value and can still be freed.
  *
  * @ingroup grid_ref
  */