fix(strategy): reject non-finite spectral_alpha in storage.generate

[philluiz2323]

Summary

storage.generate passes spectral_alpha straight to _fill_decaying_spectrum, whose weights are np.arange(1, rank+1) ** -alpha, with no finiteness check:

• alpha = nan → weights all NaN → the entire generated matrix is NaN.
• alpha = inf → weights collapse to [1, 0, 0, …] → a "rank-R decaying-spectrum" matrix silently becomes rank 1.

The sibling data_rank argument is already validated at this same public boundary; spectral_alpha was the gap.

Reproduced (CPU, current main):

storage.generate(..., fill="decaying-spectrum", spectral_alpha=nan) -> matrix all-NaN = True
storage.generate(..., fill="decaying-spectrum", spectral_alpha=inf) -> rank (data_rank was 16) = 1

Fix

Guard spectral_alpha (finite and >= 0) inside storage.generate's decaying-spectrum branch, right next to the existing data_rank guard, so every caller of the public runner/storage API is protected.

Scope note: the --spectral-alpha CLI flag that used to feed this was reverted upstream (84b6604, revert of #232), so this PR deliberately does not touch strategy/cli.py. But storage.generate remains a public function that still accepts the spectral_alpha parameter (used by the decaying-spectrum fill), so the corruption is still reachable and the boundary still needs the guard. Single file + test.

Verification (CPU — no GPU needed)

• python -m pytest tests/test_decaying_spectrum_alpha.py — generate() rejects non-finite/negative alpha for decaying-spectrum, accepts finite values (output stays finite), and ignores the knob for unrelated fills
• python -m pytest tests/test_cli_errors.py — unaffected, green (this PR no longer touches the CLI)

Scorecard

Not applicable: pure input-validation/robustness fix at the storage boundary, touching no scored path (ready-non-gpu class).

[github-actions]

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[zeokin]

Non-finite spectral_alpha silently produced an all-NaN matrix or a rank-1 collapse -- corrupt benchmark input. Closes the library boundary the CLI already guards.