Add soft_symcover! and soft_cover! refiners

[timholy]

Every other cover family paired a solver that owns a start menu with a
refiner that takes one start from the caller; the soft covers had only
the solver. The kernels for the missing half already existed as the
single-start coordinate descents the multistart drives, so the refiners
expose what was there rather than adding numerics.

This completes one rule across the whole grid: the plain form owns the
menu, so its result is a property of A, while the ! form refines the one
start it is given, so its result is a property of A and that start.
symcover!/cover! sit outside the rule because they are initializers --
they build a cover from scratch and never read the vector passed in.

The soft refiners take a start that need not cover A, since the soft
objective constrains nothing; callers should build one with
initialize_symcover(...; feasible=:none).

The shared start-validation prologues now take the caller's name, so a
bad start reports the function the user actually called instead of
always naming the *_min form.

Also state the soft tier contract in the manual: soft_* is always native
and best-effort, soft_*_min is a true minimizer that may require an
extension. The claim that the soft *_min solvers were "likewise
JuMP-backed (HiGHS for AbsLog{2})" was wrong on both counts -- AbsLog{2}
is native there and HiGHS is never used for soft covers.

Assisted-by: Claude Opus 4.8 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (285b12f) to head (0c35cd8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   98.51%   98.53%   +0.02%     
==========================================
  Files          12       12              
  Lines        2016     2052      +36     
==========================================
+ Hits         1986     2022      +36     
  Misses         30       30              

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