libghostty: expose per-cell selection state

Render-state rows already expose their selected range, but
cell-oriented C API consumers had to fetch that row range separately
and duplicate the containment check while rendering.

Add a SELECTED row-cells data kind that carries the row selection into
the row-cells wrapper and returns whether the current cell column is in
that inclusive range. The field remains separate from cell colors and
style so selection stays an explicit render overlay policy.

For performance reasons, the span-based row getter is recommended still
but this is a convenient thing to do for cell-oriented folks.

include/ghostty/vt/render.h

@@ -597,6 +597,16 @@ typedef enum GHOSTTY_ENUM_TYPED {
    *  color, in which case the caller should use whatever default foreground
    *  color it wants (e.g. the terminal foreground). */
   GHOSTTY_RENDER_STATE_ROW_CELLS_DATA_FG_COLOR = 6,
+
+  /** Whether the cell is contained within the current selection (bool).
+   *  This returns true when the cell's column is within the current row's
+   *  row-local selection range, and false otherwise. Rendering policy for
+   *  selected cells (colors, inversion, etc.) is left to the caller.
+   *
+   *  Renderers that can draw cells in spans may be more efficient querying
+   *  GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION once per row and applying that
+   *  range directly, avoiding one C API call per cell for selection state. */
+  GHOSTTY_RENDER_STATE_ROW_CELLS_DATA_SELECTED = 7,
   GHOSTTY_RENDER_STATE_ROW_CELLS_DATA_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyRenderStateRowCellsData;