Replace arrow_field with a memoizing arrow_field_ref

[emilk]

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What

ColumnDesc::arrow_field_ref() -> FieldRef, built once and thereafter just an Arc::clone. arrow_field is removed, as suggested — *desc.arrow_field_ref() clones an owned Field if you need one. It was only used in tests.

The cache is a &'static OnceLock<FieldRef>, not an inline OnceLock field. That is the part worth reviewing: the derive emits descriptors as pub const, so a cell stored by value would be materialized fresh at every use site and would never hit — a cache that silently does nothing. Holding a reference to a static means all uses of a const share one cell, and ColumnDesc stays Copy.

The derive attaches one per column:

pub const COLUMN_MAYBE_AGE: ColumnDesc<Option<i64>> =
    ColumnDesc::new("Typed", "maybe_age").with_field_cache({
        static FIELD: OnceLock<FieldRef> = OnceLock::new();
        &FIELD
    });

with_field_cache is public, so hand-written descriptors can do the same. Without it the field is rebuilt per call — correct, just not free. The cache does not take part in PartialEq or Debug.

OnceLock rather than LazyLock: LazyLock's initializer is part of its type and cannot capture, so it could not see the descriptor's name and metadata. get_or_init lets the descriptor supply the builder, so one method serves derived and hand-written descriptors alike.

Testing

New test covers repeat calls sharing an allocation, a separate copy of the const sharing it too (the case a by-value cell gets wrong), an uncached descriptor rebuilding each time, and the cache not affecting equality. Full CI job run locally on 1.95.0.

Compatibility

Breaking: arrow_field is gone, and ColumnDesc gains a private field.

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