Document IsFrattiniFree and add methods for computing it

[fingolfin]

The property IsFrattiniFree was declared in lib/grppcaut.gd, but it was undocumented and had no methods at all, so IsFrattiniFree(G) raised a "no method found" error unless something had set the property explicitly beforehand (as randiso.gi does). This documents it and makes it computable for finite groups.

The declaration moves next to FrattiniSubgroup in lib/grp.gd, together with implications in both directions: trivial, elementary abelian and finite simple groups are Frattini-free, while a finite Frattini-free p-group is elementary abelian and a finite Frattini-free nilpotent group is abelian.

The methods try to avoid computing the Frattini subgroup:

• For a finite nilpotent group the property holds if and only if all Sylow subgroups are elementary abelian, i.e. if and only if the group is abelian of squarefree exponent. This needs no subgroup computations at all.
• The generic method for finite groups first checks whether the group order is squarefree. Otherwise it uses that Phi(N) <= Phi(G) for N normal in G: applied to the Fitting subgroup, this decides many groups negatively without ever looking at maximal subgroups, and if G = F(G) it decides the question outright. Only then is Phi(G) computed.
• Conversely a known Frattini subgroup decides the property (also for groups not known to be finite), and a group known to be Frattini-free has a trivial Frattini subgroup.

The Fitting subgroup criterion is where most of the speedup comes from, e.g. GL(3,5) 13ms instead of 476ms, WreathProduct(DihedralGroup(IsPermGroup,8), AlternatingGroup(5)) 13ms instead of 70ms. When the answer is true it costs one extra Fitting subgroup computation, which is usually work the Frattini subgroup computation performs anyway.

Note that morpheus.gi and grppcext.gi still guard their use of the property with HasIsFrattiniFree. Now that it is computable, dropping that guard in morpheus.gi would let solvable groups use the much faster AutomorphismGroupFrattFreeGroup path, but that affects every solvable automorphism group computation, so it is left for a separate change.

Besides the new test file, the property was checked to agree with IsTrivial(FrattiniSubgroup(G)) for all groups in the small groups library of order at most 300, all transitive groups of degree at most 12, and all perfect groups of order at most 10^5.

AI disclosure: prepared with the help of Claude Code (Opus 5), which wrote the implementation, documentation and tests, ran the test suites and drafted this description.