fix(extract): emit inherits edge for Python parameterized bases

[Synvoya]

Bug

A Python class whose base is parameterized loses its heritage edge.
class Repo(Base[int]) and class Handler(Generic[T]) emit no
inherits edge at all, so every generic base — Generic[T],
Sequence[T], Mapping[K, V], Repository[User], list[str] — is
dropped from the graph.

Root cause

The Python branch of _extract_generic (graphify/extractors/engine.py)
iterates the superclasses list and only handles bases whose node type is
identifier:

for arg in args.children:
    if arg.type == "identifier":
        ...
        add_edge(class_nid, base_nid, "inherits", line)

A parameterized base does not parse as an identifier. tree-sitter-python
wraps it in a subscript node whose value field is the real base type and
whose [...] holds only the type parameters:

class_definition "class Repo(Base[int])"
  superclasses: argument_list
    subscript "Base[int]"
      value: identifier "Base"   <-- the actual base
      subscript: integer "int"

The subscript node never matched the identifier check, so the base fell
through and the edge was silently dropped.

Minimal repro

class Base: pass
class Repo(Base[int]): pass       # inherits Repo -> Base  (dropped)
class Multi(Mixin, Base[str]): pass  # Mixin kept, Base dropped

extract_python before this change yields only the bare-identifier edges;
every subscript base is missing.

Fix

Unwrap a subscript base to its value field before the identifier check:

if base_node.type == "subscript":
    base_node = base_node.child_by_field_name("value") or base_node
if base_node.type == "identifier":
    ...

• Bare-identifier bases (class C(Base)) are unaffected.
• Multiple bases mixing plain and parameterized (class C(A, B[T])) now
  keep both edges.
• Qualified (attribute) bases such as pkg.mod.Base are intentionally left
  out of scope here — those are the domain of the cross-file symbol resolver
  (issue AST extractor misses cross-file inheritance and produces ghost-duplicate nodes for cross-file type references #1186 / PR Fix cross-file inheritance and ghost type references in AST extractor #1231), which resolves an imported base name back to its
  canonical cross-file definition. This change is confined to node-type
  coverage in the immediate AST-traversal path and does not touch that
  pipeline.

Test

tests/test_languages.py::test_python_parameterized_base_inherits_edge
asserts Repo -> Base and Multi -> Base (subscript bases) plus
Multi -> Mixin (bare-identifier regression guard).

• Before: fails — inherits == {('Multi', 'Mixin')}; both subscript
  bases dropped.
• After: passes.

Full tests/test_languages.py suite: 315 passed, 13 skipped (unchanged;
skips are the optional tree-sitter-dm extra).