feat(core): add headerCursor override for column headers

[dogfootruler-kr]

This PR adds a per-column header cursor override.

Why
Allows consumers to customize cursor feedback on header hover (e.g., help, grab, context-menu) without affecting cell cursors.

Notes

• No behavioral change unless headerCursor is provided.
• Lints pass locally.

Example

const columns = [
  { title: "Name", width: 200, headerCursor: "help" },
  { title: "Status", id: "status", headerCursor: "grab" },
];