Port tensor4all-aci elementwise API and benchmarks

[shinaoka]

Summary

• Port tensor4all-aci as the Rust counterpart of AlternatingCrossInterpolation.jl elementwise ACI for tensor trains.
• Add the public ACI API surface: elementwise, elementwise_batched, ElementwiseBatch, AciOptions, AciResult, typed errors, README examples, and doctested usage.
• Wire ACI local updates through tensor4all-simplett, tensor4all-tcicore, and tensor4all-tensorbackend, including deterministic tests against dense oracles.
• Add Rust/Julia benchmark runners and saved benchmark results for elementwise ACI, local-step bucket timing, MatrixLUCI, and standalone MatrixLU.
• Align the Rust convergence check with Julia's convergencecriterion shape and reject min_iters = 0 as an invalid option.

Performance Work Included

• Reduce Matrix <-> tenferro TypedTensor conversion overhead with owned matrix and batched-GEMM paths.
• Add an owned MatrixLUCI factorization path for ACI local matrices.
• Optimize rrLU hot loops over prevalidated column-major slices, keeping unchecked indexing localized in small helpers.
• Store the durable dense-loop lesson in REPOSITORY_RULES.md: validate ranges once, then iterate over column-major slices in hot paths.

Benchmark Snapshot

L=16 fixed-sweep local-step medians, Rust built with tenferro-system-blas against Homebrew OpenBLAS:

chi	Rust total ms	Julia total ms	Rust MatrixLUCI ms	Julia MatrixLUCI ms	rank
16	1.841535	2.115942	1.259810	1.233481	33
32	3.330730	4.085233	2.209271	2.414964	46
64	7.975667	9.584417	4.708000	5.213416	63
128	15.889415	17.230575	7.987332	8.276169	76

Saved details are in benchmarks/results/2026-05-22-aci-local-step-l16-openblas.md and docs/plans/2026-05-22-aci-rust-api-handoff.md.

Validation

• git diff --check
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --release -p tensor4all-tensorbackend --lib
• cargo test --release -p tensor4all-tcicore
• cargo test --release -p tensor4all-aci
• cargo test --doc --release -p tensor4all-tensorbackend
• cargo test --doc --release -p tensor4all-aci
• cargo test --release -p tensor4all-aci --no-default-features --features tenferro-system-blas with RUSTFLAGS, RUSTDOCFLAGS, and DYLD_LIBRARY_PATH pointed at Homebrew OpenBLAS

Notes

• MatrixLU standalone still trails Julia on the saved Hilbert microbenchmark; this is tracked separately because the ACI local MatrixLUCI bucket is now close under the L=16 fixture.
• Upstream AlternatingCrossInterpolation.jl currently lists Marc Ritter and contributors in its Project.toml/LICENSE, while the paper citation is Marc Ritter.