Declare the extension hooks public and document their contracts

[timholy]

foreach_support and foreach_support_sym are how a cover algorithm reads a
matrix: specializing them is what lets a storage type be covered in time
proportional to its support, as the bundled SparseArrays extension does.
AbstractCoverPenalty is the supertype of the exported penalties and the
first-argument slot of most of the API. All three were reachable and
relied upon, but none declared its status, so users had no way to tell a
supported extension point from an internal that could move.

Each now states what an implementer must guarantee. Two points that the
code enforced only by convention are written down:

• foreach_support_sym requires A to be symmetric in value, not merely
  square. It cannot check this cheaply and does not try; the precondition
  is what makes reporting one member of each index pair sufficient.
• Subtyping AbstractCoverPenalty buys cover_objective and nothing more.
  The solvers dispatch on the concrete built-in penalties, so a custom
  penalty can score a cover but cannot be minimized. Saying so beats
  letting a MethodError be the documentation.

public is parsed as a keyword only from Julia 1.11, so the declaration
goes through a VERSION gate rather than a new Compat dependency.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.51%. Comparing base (1c194d8) to head (d86e994).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          12       12           
  Lines        2016     2016           
=======================================
  Hits         1986     1986           
  Misses         30       30           

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