add orphan prevention tests

tests/suite/layout/grid/headers.typ

@@ -250,6 +250,17 @@
   ..([Test], [Test], [Test]) * 20
 )
 
+--- grid-header-non-repeating-orphan-prevention ---
+#set page(height: 5em)
+#v(2em)
+#grid(
+  grid.header(repeat: false)[*Abc*],
+  [a],
+  [b],
+  [c],
+  [d]
+)
+
 --- grid-header-empty ---
 // Empty header should just be a repeated blank row
 #set page(height: 12em)

tests/suite/layout/grid/subheaders.typ

@@ -197,6 +197,8 @@
 )
 
 --- grid-subheaders-repeat-replace-short-lived ---
+// No orphan prevention for short-lived headers
+// (followed by replacing headers).
 #set page(height: 8em)
 #grid(
   grid.header(
@@ -345,6 +347,56 @@
   grid.cell(x: 0)[done.]
 )
 
+--- grid-subheaders-short-lived-no-orphan-prevention ---
+// No orphan prevention for short-lived headers.
+#set page(height: 8em)
+#v(5em)
+#grid(
+  grid.header(
+    level: 2,
+    [b]
+  ),
+  grid.header(
+    level: 2,
+    [c]
+  ),
+  [d]
+)
+
+--- grid-subheaders-repeating-orphan-prevention ---
+#set page(height: 8em)
+#v(4.5em)
+#grid(
+  grid.header(
+    repeat: true,
+    level: 2,
+    [L2]
+  ),
+  grid.header(
+    repeat: true,
+    level: 4,
+    [L4]
+  ),
+  [a]
+)
+
+--- grid-subheaders-non-repeating-orphan-prevention ---
+#set page(height: 8em)
+#v(4.5em)
+#grid(
+  grid.header(
+    repeat: false,
+    level: 2,
+    [L2]
+  ),
+  grid.header(
+    repeat: false,
+    level: 4,
+    [L4]
+  ),
+  [a]
+)
+
 --- grid-subheaders-alone ---
 #table(
   table.header(