coerce Arrow BigInt to Number

[mbostock]

Ref. https://github.com/observablehq/plot/pull/2155#pullrequestreview-2266406998

Note that in the case of DuckDB-Wasm, we default config.query.castBigIntToDouble to true, so we shouldn’t see BigInt in practice with DuckDB, but we might still see it when loading Arrow or Parquet files.

[Fil]

Sounds good. I'll work on a unit test.

[Fil]

I've added a unit test, however it reveals some other issues. I'll comment inline.