terminal: cell returns empty for Kitty placeholder

So we don't render the replacement char

src/terminal/Terminal.zig

@@ -379,7 +379,11 @@ pub fn print(self: *Terminal, c: u21) !void {
                 }
             }
 
-            log.debug("c={x} grapheme attach to left={}", .{ c, prev.left });
+            log.debug("c={X} grapheme attach to left={} primary_cp={X}", .{
+                c,
+                prev.left,
+                prev.cell.codepoint(),
+            });
             self.screen.cursorMarkDirty();
             try self.screen.appendGrapheme(prev.cell, c);
             return;

src/terminal/kitty/graphics.zig

@@ -21,6 +21,7 @@ pub usingnamespace @import("graphics_exec.zig");
 pub usingnamespace @import("graphics_image.zig");
 pub usingnamespace @import("graphics_storage.zig");
 pub const diacritics = @import("graphics_diacritics.zig");
+pub const placeholder = diacritics.placeholder;
 
 test {
     @import("std").testing.refAllDecls(@This());

src/terminal/kitty/graphics_diacritics.zig

@@ -2,6 +2,9 @@ const std = @import("std");
 const assert = std.debug.assert;
 const testing = std.testing;
 
+/// Codepoint for the unicode placeholder character.
+pub const placeholder: u21 = 0x10EEEE;
+
 /// Get the row/col index for a diacritic codepoint.
 pub fn get(cp: u21) ?usize {
     return std.sort.binarySearch(u21, cp, diacritics, {}, (struct {

src/terminal/page.zig

@@ -8,6 +8,7 @@ const posix = std.posix;
 const fastmem = @import("../fastmem.zig");
 const color = @import("color.zig");
 const hyperlink = @import("hyperlink.zig");
+const kitty = @import("kitty.zig");
 const sgr = @import("sgr.zig");
 const style = @import("style.zig");
 const size = @import("size.zig");
@@ -1673,11 +1674,18 @@ pub const Cell = packed struct(u64) {
         return @as(u64, @bitCast(self)) == 0;
     }
 
+    /// Returns true if this cell represents a cell with text to render.
+    ///
+    /// Cases this returns false:
+    ///   - Cell text is blank
+    ///   - Cell is styled but only with a background color and no text
+    ///   - Cell has a unicode placeholder for Kitty graphics protocol
     pub fn hasText(self: Cell) bool {
         return switch (self.content_tag) {
             .codepoint,
             .codepoint_grapheme,
-            => self.content.codepoint != 0,
+            => self.content.codepoint != 0 and
+                self.content.codepoint != kitty.graphics.placeholder,
 
             .bg_color_palette,
             .bg_color_rgb,
@@ -1709,7 +1717,8 @@ pub const Cell = packed struct(u64) {
         return self.style_id != style.default_id;
     }
 
-    /// Returns true if the cell has no text or styling.
+    /// Returns true if the cell has no text or styling. This also returns
+    /// true if the cell represents a Kitty graphics unicode placeholder.
     pub fn isEmpty(self: Cell) bool {
         return switch (self.content_tag) {
             // Textual cells are empty if they have no text and are narrow.
@@ -2641,3 +2650,12 @@ test "Page verifyIntegrity zero cols" {
         page.verifyIntegrity(testing.allocator),
     );
 }
+
+test "Cell isEmpty for kitty placeholder" {
+    var c: Cell = .{
+        .content_tag = .codepoint_grapheme,
+        .content = .{ .codepoint = kitty.graphics.placeholder },
+    };
+    try testing.expectEqual(@as(u21, kitty.graphics.placeholder), c.codepoint());
+    try testing.expect(c.isEmpty());
+}