refactor!: Rename the decoded result types to the Tensor family

[kylebarron]

@d-v-b and I got on a call and decided on some names, as we wrote down here.

In particular, our current DecodedArray becomes Tensor, a type alias that is a union of fixed and variable-length in-memory n-d arrays.

In the future, I would like to explore merging FixedLengthTensor and OptionalFixedLengthTensor into a single class. But that would be a bit messy because the __array__ would sometimes convert to a plain array and other times convert to a masked array.

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Written by Claude

Supersedes #136. That PR renamed the DecodedArray alias to ArrayData; this takes a different direction, and branches fresh off main instead (#136 had gone stale against #145–#150 and #160–#168).

A read returns a Tensor. The four concrete classes are named after the two axes that actually decide the layout: fixed- vs variable-length, and optional (masked) vs not.

Before	After
Tensor (class)	FixedLengthTensor
VariableArray	VariableLengthTensor
MaskedTensor	OptionalFixedLengthTensor
MaskedVariableArray	OptionalVariableLengthTensor
DecodedArray (alias)	Tensor

Tensor: TypeAlias = (
    FixedLengthTensor
    | VariableLengthTensor
    | OptionalFixedLengthTensor
    | OptionalVariableLengthTensor
)

Rust side

• The Py-prefixed wrappers take the matching names.
• The internal enum becomes PyTensor (it implements IntoPyObject, so it takes the Py prefix), with variants that mirror the ArrayBytes layouts that produce them: Fixed, Variable, OptionalFixed, OptionalVariable.
• src/data/tensor.rs becomes src/data/fixed.rs, to pair with variable.rs.
• repr::decoded_array_repr becomes repr::tensor_repr.
• PyTensorBuffer keeps its name. It is an internal buffer-protocol helper and is not exported to Python.

Scope

Rename only. No behavior changes.

Follow-up

We want to prototype collapsing the four classes into two, with nullability carried as an optional mask on FixedLengthTensor / VariableLengthTensor rather than as separate Optional* classes. That is a separate PR; this one lands the naming first. The Optional names may get another look then.

Verification

• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo test --all-features — 28 passed
• nightly cargo fmt --check (with the CI config) — clean
• pytest — 325 passed, 1 xfailed
• ruff check / ruff format --check — clean
• pydoclint on the .pyi stubs — no violations
• mkdocs build --strict — clean

🤖 Generated with Claude Code