inheritors_of can report a same-named class from another namespace as an inheritor

[merlincat11]

Split out of #939 (review discussion) to keep that PR scoped to the parser bug in #935.

Problem

inheritors_of resolves the requested node, and when exact qualified-target lookup finds
nothing it falls back to the node's plain name. That fallback matches edges by bare
target plus source language, which does not establish namespace or package identity.

A/GenericBase.cs:  namespace A; class GenericBase<T> {}
B/GenericBase.cs:  namespace B; class GenericBase<T> {}
B/Child.cs:        namespace B; class Child : GenericBase<string> {}

Querying the A declaration by its qualified graph name returns B.Child, with no
caveat attached:

$ code-review-graph query inheritors_of /repo/A/GenericBase.cs::GenericBase
count: 1  results: ['Child']   confidence: None

Not specific to generics

Reproduces on main with plain class Child : PlainBase {} and two same-named
PlainBase declarations. This is the bare-name fallback, not the generic handling.

Why it matters

#935 objected to the graph making unjustified affirmative claims about inheritance. A
false positive asserted without qualification is the same failure pointing the other way.

Options

1. Resolve the base name against namespace/package/import context before treating the
   relationship as resolved — the real fix, and the one worth doing.
2. Attach a caveat when the fallback matched an ambiguous bare name, so the result is
   returned but not asserted. A prototype existed in fix(parser): resolve generic inheritance targets #939 (restricting the ambiguity
   count to Class/Type kinds) and was removed to keep that PR scoped.

Option 2 is a mitigation; option 1 is the fix.