Load NautyTracesInterface support as an extension

[fingolfin]

Until now the nauty support (DIGRAPHS_NautyAvailable, NAUTY_DATA) was read only if NautyTracesInterface was already marked for loading when Digraphs got loaded. Whether the feature is available therefore depended on load order: LoadPackage("Digraphs"); LoadPackage("NautyTracesInterface"); silently left it out, and the only remedy was restarting GAP and loading the packages the other way round.

This PR declares that code in the Extensions component of PackageInfo.g instead (see gap-system/gap#5760 and the section "Extensions Provided by a Package" in the GAP reference manual). GAP reads an extension as soon as all packages it needs are loaded, in whichever order that happens.

Extensions require GAP 4.13 (released 2024), so this raises the minimal GAP version to 4.13; GAP 4.11 and 4.12 are dropped from CI.

Tested: loading Digraphs with OnlyNeeded and then NautyTracesInterface (and GRAPE), and the reverse order, both provide the feature; tst/teststandard.g passes with and without suggested packages.

Before, loading NautyTracesInterface after Digraphs left nauty unusable, e.g. NautyAutomorphismGroup returned fail.

There are two further commits:

• Stop binding GRAPE names when GRAPE is absent: without GRAPE, Digraphs bound IsGraph, Vertices and Adjacency itself, so loading GRAPE afterwards printed warnings such as BIND_GLOBAL: variable `Adjacency' already has a value. These functions are now looked up at runtime.
• Remove code for GAP versions before 4.13: the kernel module and UserHomeExpand fallbacks, the IsKernelExtensionAvailable workaround in PackageInfo.g, and a CompareVersionNumbers branch in oper.gi.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (8d49e9a) to head (5158fa1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #950      +/-   ##
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+ Coverage   97.45%   97.47%   +0.02%     
==========================================
  Files          50       51       +1     
  Lines       21189    21187       -2     
  Branches      639      639              
==========================================
+ Hits        20649    20652       +3     
+ Misses        475      470       -5     
  Partials       65       65              

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