`open_grid` on large SCRIP grids OOMs: corner dedup ignores `chunks=`

[rajeeja]

• SCRIP stores one (lon, lat) per face-corner, so the corner table is n_face × n_corners rows.
• _to_ugrid dedups it with Polars, which needs the whole table resident: 53.6 GiB of float64 for a 300M-element np4 grid, before the join copy.
• The reader never dispatches on chunks=, so large SCRIP files OOM no matter how they're opened.
• This blocks the 63 GB CONUS-RRM np4 grid served through the uxarray-mcp-server, which already passes chunks="auto" and still OOMs on the worker.