chore: migrate internal tests/benches off deprecated row-major constructors

[isPANN]

What

Internal tests, doc examples, and the CUDA AD benchmark were calling the deprecated row-major convenience constructors, so the crate emitted 55+ deprecation warnings against its own API. Migrate every internal caller to the native column-major path — data hand-transposed so the logical matrices are unchanged (every assertion still holds):

deprecated (kept public)	→ migrated to	where
Mat::from_row_major	from_col_major	CPU mat/*
GpuMatrix::from_host_row_major	from_host	CUDA input (lib.rs tests, bench)
argmax_to_host_row_major	argmax_to_host	CUDA argmax output (bench)

The deprecated methods stay public as a perf hint for downstream consumers; a few #[allow(deprecated)] tests keep them covered. Also dropped the bogus since = "0.4.0" from both deprecation attributes (crate is 0.2.0, and since doesn't gate the warning anyway — it fires on every use immediately).

Result

0 deprecation warnings (was 55+), 0 total workspace warnings.

Verification

• cargo test -p tropical-gemm → 275 unit + 24 doctests pass
• maturin develop + pytest → 152 pass (CPU)
• CUDA 56/56 on HKUST-GZ A40 (Slurm job 9800403)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.29%. Comparing base (cfa56d4) to head (c053d45).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files          19       19           
  Lines         918      918           
=======================================
  Hits          884      884           
  Misses         34       34           

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