Keep gramcover accurate when squares leave range

[timholy]

gramcover(A) and gramcover(A, w) sum abs(w[i]) * A[i,j]^2 per column and take one square root. An entry below about 1e-162 squares to zero and one above about 1e154 squares to Inf, so a column whose 2-norm is an ordinary number could get a scale of 0 or Inf: gramcover([1e-170 1.0]) returned [0.0, 1.0], which does not cover A'*A.

The plain sum of squares is kept as the first pass. A column whose sum is below floatmin, infinite, or NaN is accumulated again as a running hypot over its terms sqrt(abs(w[i])) * abs(A[i,j]). The scale is then accurate to the documented O(n_j * eps) whenever the largest term in the column is at least floatmin, and is Inf only when the norm exceeds floatmax. The existing inflation factor bounds the rounding of both forms.

The weight now enters each term as sqrt(abs(w[i])) before squaring, so weighted results change in the last bits. Entries are converted to floating point before squaring, which keeps the squares of large integer entries from wrapping.

When no column needs the second pass the time is unchanged: 1.43 ms for a dense 2000x500 matrix and 0.14 ms for a sparse matrix with 100000 nonzeros. When every column needs it, the sparse case takes 1.3 ms. Using hypot for every entry instead takes 13.6 ms and 1.1 ms.

Assisted-by: Claude Opus 5.5 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (b80567c) to head (7d716bf).

Additional details and impacted files

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##             main      #87   +/-   ##
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  Coverage   98.79%   98.79%           
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  Files          23       23           
  Lines        6286     6302   +16     
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+ Hits         6210     6226   +16     
  Misses         76       76           

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