Run post_init for the class being instantiated

[ESultanik]

Closes #149

DataClassNode.__init__ called post_init() only on the entries of _DATA_CLASS_ANCESTORS, which excludes the
class being constructed. A subclass that used the documented extension point instead of overriding __init__ never
had its callback run, and nothing reported the omission: the callback began firing only once some other class
subclassed it. The ancestor list was also in __mro__ order, most derived first, so ancestor callbacks ran in the
reverse of constructor order, contradicting the base-up initialization the post_init() docstring described.

Each class in the hierarchy that defines its own post_init is now recorded at class-definition time, least derived
first and ending with the class itself. Selecting on __dict__ keeps an implementation that a subclass inherits
without overriding from running once per inheriting class, so subclasses of a data class still get exactly one call.

docs/builders.rst carried a note documenting the old behavior as a limitation; it is replaced with the inherited
implementation rule.

No DataClassNode subclass in the package defines post_init (graphtage/ast.py and graphtage/pydiff.py use
__init__ overrides), so no in-tree node changes behavior.

Validation

Three tests in test/test_dataclasses.py were confirmed to fail against the unfixed code and pass after the fix:

• test_post_init_runs_once_per_implementation asserts the call order and count for a three-level hierarchy where the
  middle class inherits its post_init (expected [("Base", "Derived"), ("Derived", "Derived")]; the unfixed code
  produced [] for the first instantiation).
• test_post_init_runs_for_direct_subclass covers the UnquotedName example from the docs.
• test_inheritance was strengthened: both Foo.post_init and Bar.post_init set distinct flags, so the assertion no
  longer passes when only the ancestor's callback runs. It previously passed by accident.

The reproducer from the issue now prints [('Node1', 'Node1')] for the direct instantiation, and a diamond hierarchy
calls each implementation once in base-first order.

Local CI:

• ruff check graphtage test docs bindist: passed
• pytest: 142 passed
• make -C docs html SPHINXOPTS="-W --keep-going": build succeeded
• uv lock --check: fails in this sandbox on an unmodified checkout of master as well, because of a global
  exclude-newer timestamp. No dependency files were touched.

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