Add per-crate prelude modules to eliminate the E0599 missing-trait failure mode

[shinaoka]

Problem

Core methods in this workspace live on traits, not inherent impls: sum() and rank() on AbstractTensorTrain (crates/tensor4all-simplett/src/traits.rs), contract_pair on TensorContractionLike (crates/tensor4all-core/src/tensor_like.rs). No crate ships a prelude module (pub mod prelude has zero hits across crates/).

Consequence: the most likely first-try failure for a new user or coding agent is error[E0599]: no method named 'sum' found after forgetting a trait import it had no reason to guess. Every quickstart in the repo works around this with an import list:

use tensor4all_simplett::{AbstractTensorTrain, CompressionOptions, TensorTrain};

tensor4all-interpolativeqtt partially compensates by re-exporting (src/lib.rs:47: pub use tensor4all_simplett::{AbstractTensorTrain, TensorTrain};), but this courtesy is not systematic.

Proposed direction

Add a conventional prelude module to each user-facing crate (tensor4all-core, tensor4all-simplett, tensor4all-treetn, tensor4all-itensorlike, tensor4all-tensorci, tensor4all-quanticstci, tensor4all-aci, tensor4all-interpolativeqtt), re-exporting that crate's public traits plus the handful of types needed to call them, so user code starts with:

use tensor4all_simplett::prelude::*;

Notes:

• ndarray, rayon, and std establish the convention; glob-importing a module designed for it is accepted style.
• Update README and book quickstarts to use the preludes (the doctest harness keeps them honest).
• Reference implementation: [Feature]: Add per-crate prelude modules to eliminate the E0599 missing-trait failure mode tenferro-rs#1612, closed by feat: add crate-local preludes tenferro-rs#1635 (crate-local preludes across the tenferro workspace).

Non-goals

A facade crate; renaming or merging traits; deprecating direct imports.

Acceptance criteria

• Each user-facing crate has a documented prelude covering its public traits.
• README and getting-started quickstarts compile with prelude imports only.