terminal: fix printCell corrupting previous row when overwriting wide char

printCell, when overwriting a wide cell with a narrow cell at x<=1 and
y>0, unconditionally sets the last cell of the previous row to .narrow.
This is intended to clear a spacer_head left by a wrapped wide char, but
the cell could be a spacer_tail if a wide char fit entirely on the
previous row. Setting a spacer_tail to .narrow orphans the preceding
.wide cell, which later causes an integrity violation in insertBlanks
(assert that the cell after a .wide is .spacer_tail).

Fix by guarding the assignment so it only fires when the previous row's
last cell is actually a .spacer_head. The same fix is applied in both
the .wide and .spacer_tail branches of printCell.

Found by AFL++ stream fuzzer.

src/terminal/Terminal.zig

@@ -723,9 +723,14 @@ fn printCell(
                     self.screens.active.cursor.page_row,
                     spacer_cell[0..1],
                 );
+
+                // If we're near the left edge, a wide char may have
+                // wrapped from the previous row, leaving a spacer_head
+                // at the end of that row. Clear it so the previous row
+                // doesn't keep a stale spacer_head.
                 if (self.screens.active.cursor.y > 0 and self.screens.active.cursor.x <= 1) {
                     const head_cell = self.screens.active.cursorCellEndOfPrev();
-                    head_cell.wide = .narrow;
+                    if (head_cell.wide == .spacer_head) head_cell.wide = .narrow;
                 }
             },
 
@@ -744,9 +749,13 @@ fn printCell(
                     self.screens.active.cursor.page_row,
                     wide_cell[0..1],
                 );
+                // If we're near the left edge, a wide char may have
+                // wrapped from the previous row, leaving a spacer_head
+                // at the end of that row. Clear it so the previous row
+                // doesn't keep a stale spacer_head.
                 if (self.screens.active.cursor.y > 0 and self.screens.active.cursor.x <= 1) {
                     const head_cell = self.screens.active.cursorCellEndOfPrev();
-                    head_cell.wide = .narrow;
+                    if (head_cell.wide == .spacer_head) head_cell.wide = .narrow;
                 }
             },
 
@@ -3341,6 +3350,44 @@ test "Terminal: print over wide char at 0,0" {
     try testing.expect(!t.isDirty(.{ .screen = .{ .x = 0, .y = 1 } }));
 }
 
+test "Terminal: print over wide char at col 0 corrupts previous row" {
+    // Crash found by AFL++ fuzzer (afl-out/stream/default/crashes/id:000002).
+    //
+    // printCell, when overwriting a wide cell with a narrow cell at x<=1
+    // and y>0, sets the last cell of the previous row to .narrow — even
+    // when that cell is a .spacer_tail rather than a .spacer_head. This
+    // orphans the .wide cell at cols-2.
+    const alloc = testing.allocator;
+    var t = try init(alloc, .{ .cols = 10, .rows = 3 });
+    defer t.deinit(alloc);
+
+    // Fill rows 0 and 1 with wide chars (5 per row on a 10-col terminal).
+    for (0..10) |_| try t.print(0x4E2D);
+
+    // Move cursor to row 1, col 0 (on top of a wide char) and print a
+    // narrow character. This triggers printCell's .wide branch which
+    // corrupts row 0's last cell: col 9 changes from .spacer_tail to
+    // .narrow, orphaning the .wide at col 8.
+    t.setCursorPos(2, 1);
+    try t.print('A');
+
+    // Row 1, col 0 should be narrow (we just overwrote the wide char).
+    {
+        const list_cell = t.screens.active.pages.getCell(.{ .screen = .{ .x = 0, .y = 1 } }).?;
+        try testing.expectEqual(Cell.Wide.narrow, list_cell.cell.wide);
+    }
+    // Row 0, col 8 should still be .wide (the last wide char on the row).
+    {
+        const list_cell = t.screens.active.pages.getCell(.{ .screen = .{ .x = 8, .y = 0 } }).?;
+        try testing.expectEqual(Cell.Wide.wide, list_cell.cell.wide);
+    }
+    // Row 0, col 9 must remain .spacer_tail to pair with the .wide at col 8.
+    {
+        const list_cell = t.screens.active.pages.getCell(.{ .screen = .{ .x = 9, .y = 0 } }).?;
+        try testing.expectEqual(Cell.Wide.spacer_tail, list_cell.cell.wide);
+    }
+}
+
 test "Terminal: print over wide spacer tail" {
     var t = try init(testing.allocator, .{ .rows = 5, .cols = 5 });
     defer t.deinit(testing.allocator);