Handle disconnected TreeTN fit contractions

[shinaoka]

Summary

• Insert dimension-1 dummy contraction links for nodewise outer-product spectators in partial_contract so compatible TreeTN topologies can contract without falling back to dense materialization.
• Make TreeTN fit environment contractions component-aware, contracting disconnected local tensor components separately and combining them by outer product.
• Add regression coverage for partial contraction spectator nodes and fit contractions whose local site space contracts away.

Rationale

The QuanticsNEGF port exposed TreeTN topologies where local node tensors can share topology but have no contractable index at a matching node. Julia-style tensor operations treat this as a valid outer-product spectator case; the previous TreeTN path could fail with disconnected contraction errors.

Validation

Run with RAYON_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 OMP_NUM_THREADS=1 MKL_NUM_THREADS=1 VECLIB_MAXIMUM_THREADS=1:

• cargo test -p tensor4all-treetn --release -- --nocapture
• cargo clippy -p tensor4all-treetn --all-targets --release -- -D warnings
• Downstream quanticsnegf-rs-2: cargo clippy --workspace --all-targets --release -- -D warnings
• Downstream quanticsnegf-rs-2: cargo test --release -- --nocapture