Add AnyList<L>: one logical type for any list encoding

[emilk]

Arrow has five physically different ways to store the same logical thing — a column of lists of L: List, LargeList, ListView, LargeListView, and FixedSizeList. They already share the same Datatype::Value (ListValue<'a, L>) and Owned (Vec<L::Owned>) — only the physical encoding differs.

AnyList<L> unifies them: Column<AnyList<L>> accepts whichever encoding it is handed at parse time and reads them all uniformly, dispatching at runtime over an internal enum of the per-encoding typed representations.

let column = Column::<AnyList<Utf8>>::try_from(array)?;
// `array` may be List / LargeList / ListView / LargeListView / FixedSizeList:
for list in &column {
    for s in list { /* &str */ }
}

Design

• AnyTypedList<L> is an enum over the five typed reps. Element access (value/is_null/iterate) delegates to whichever encoding — reusing the existing Datatype impls for the four variable-length encodings, and reading the per-row length at runtime for FixedSizeList (whose size is a const generic the dynamic type can't carry).
• An enum, not a dyn trait object: no boxing, and the uniform Value type composes (nesting, Option, etc.).
• matches() accepts all five datatypes with a matching inner L; build/from_values emits the canonical List.
• Clone is hand-written (deriving would add a spurious L: Clone bound; only L::Typed: Clone is needed) — consistent with TypedList & co.

Verification

• cargo clippy --all-features --all-targets clean, cargo fmt --all applied
• cargo test --all-features green, incl. a new any_list_columns test: build → canonical List; try_from each of the five encodings read uniformly; non-list rejected; item-nullability enforced regardless of encoding; null rows via Option<AnyList<…>>
• cargo doc --document-private-items -D warnings clean

🤖 Generated with Claude Code