Rename TensorDynLen to IdxTensor for ITensor compatibility

[shinaoka]

Motivation

TensorDynLen is the core tensor type in tensor4all-rs, analogous to ITensor's ITensor type. The current name describes implementation details (dynamic-length indices) rather than what the type conceptually represents. To improve discoverability and emphasize compatibility with the ITensor ecosystem, we should rename it to IdxTensor — a name that highlights the index-based tensor semantics shared with ITensor.

Proposal

1. Rename pub struct TensorDynLen → pub struct IdxTensor in crates/tensor4all-core/src/defaults/tensordynlen.rs
2. Keep pub type TensorDynLen = IdxTensor; as a deprecated type alias for backward compatibility during the transition
3. Update all internal usage (library code, tests, examples, docs) to use IdxTensor
4. Update C API and language bindings as needed

Scope

• 152 files, ~1926 occurrences of TensorDynLen across the workspace
• Struct definition: crates/tensor4all-core/src/defaults/tensordynlen.rs
• Re-exported from crates/tensor4all-core/src/lib.rs

Notes

• Since the project is in early development with no backward compatibility guarantees, the alias is mainly for a smooth transition period and can be removed later.
• Consider also renaming the source file from tensordynlen.rs to idx_tensor.rs.

[shinaoka]

Update on scope: given the project is still in early development and the current downstream usage appears to be minimal (only one create call on the downstream side), we should prefer a one-shot rename instead of keeping a compatibility alias.

Suggested direction:

• rename TensorDynLen to IdxTensor
• do not keep pub type TensorDynLen = IdxTensor
• update all internal uses, docs, tests, examples, C API references, and language bindings in the same change
• rename the source file/module as well if appropriate

This matches the repo policy of removing deprecated names immediately rather than carrying transitional API baggage.