PageList: preserve size.cols in adjustCapacity after column shrink

When columns shrink during resize-without-reflow, page.size.cols is
updated but page.capacity.cols retains the old larger value. When
adjustCapacity later runs (e.g., to expand style/grapheme storage),
it was creating a new page using page.capacity which has the stale
column count, causing size.cols to revert to the old value.

This caused a crash in render.zig where an assertion checks that
page.size.cols matches PageList.cols.

Fix by explicitly copying page.size.cols to the new page after
creation, matching how size.rows is already handled.

Amp-Thread-ID: https://ampcode.com/threads/T-976bc49a-7bfd-40bd-bbbb-38f66fc925ff
Co-authored-by: Amp <amp@ampcode.com>

src/terminal/PageList.zig

@@ -2608,7 +2608,9 @@ pub fn adjustCapacity(
     errdefer self.destroyNode(new_node);
     const new_page: *Page = &new_node.data;
     assert(new_page.capacity.rows >= page.capacity.rows);
+    assert(new_page.capacity.cols >= page.capacity.cols);
     new_page.size.rows = page.size.rows;
+    new_page.size.cols = page.size.cols;
     try new_page.cloneFrom(page, 0, page.size.rows);
 
     // Fix up all our tracked pins to point to the new page.
@@ -6257,6 +6259,39 @@ test "PageList adjustCapacity to increase hyperlinks" {
     }
 }
 
+test "PageList adjustCapacity after col shrink" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 10, 2, 0);
+    defer s.deinit();
+
+    // Shrink columns - this updates size.cols but not capacity.cols
+    try s.resize(.{ .cols = 5, .reflow = false });
+    try testing.expectEqual(5, s.cols);
+
+    {
+        const page = &s.pages.first.?.data;
+        // capacity.cols is still 10, but size.cols should be 5
+        try testing.expectEqual(5, page.size.cols);
+        try testing.expect(page.capacity.cols >= 10);
+    }
+
+    // Now adjust capacity (e.g., to increase styles)
+    // This should preserve the current size.cols, not revert to capacity.cols
+    _ = try s.adjustCapacity(
+        s.pages.first.?,
+        .{ .styles = std_capacity.styles * 2 },
+    );
+
+    {
+        const page = &s.pages.first.?.data;
+        // After adjustCapacity, size.cols should still be 5, not 10
+        try testing.expectEqual(5, page.size.cols);
+        try testing.expectEqual(5, s.cols);
+    }
+}
+
 test "PageList pageIterator single page" {
     const testing = std.testing;
     const alloc = testing.allocator;