scaling.wvm: return unchanged signal for a single site

[bfs2021]

Problem

pvlib.scaling.wvm returns an entirely NaN output for a layout with one coordinate pair. See #2895 for the full report, including the float-precision analysis (0.29 * 100 == 28.999...-style rounding is a separate symptom of the same rounder).

Root cause

With a single position, scipy.spatial.distance.pdist returns an empty distance array, so n_pairs = 0 and the effective-size objective has two roots (x=0 and x=1). The optimizer converges to n_dist = 0, which makes the Eq. 6 denominator 2 * sum(rho) + n_dist equal to zero — dividing by zero yields NaN for every timescale, and the reconstructed signal is all NaN.

Fix

_compute_vr now returns a variability reduction factor of 1 for every timescale when positions contains a single site. Per the WVM equations, one site has no pairwise correlations and therefore no variability reduction, so wvm passes the input signal through unchanged — the behavior the issue describes as expected.

Testing

• test_compute_vr_single_site: _compute_vr with one position returns all ones for every timescale.
• test_wvm_single_site: wvm with one position returns the input signal unchanged (previously all NaNs).
• pytest tests/test_scaling.py → 18 passed (16 existing + 2 new), Python 3.12, Windows.

Fixes #2895

Checklist

• Closes BUG: scaling.wvm returns all NaNs for a single-site layout #2895
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• Adds a "what's new" entry in docs/sphinx/source/whatsnew/v0.16.2.rst (:issue:2895, :pull:2896, by :ghuser:bfs2021)
• New code is fully documented. (no public API change; the single-site early return is commented in code with a link to this issue)
• Pull request is nearly complete and ready for detailed review.

[github-actions]

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