Cythonize _graph/_graph_builder (move from pure Python to .pyx)

Move the GraphBuilder/Graph/GraphCompleteOptions/GraphDebugPrintOptions
implementation out of _graph/__init__.py into _graph/_graph_builder.pyx
so it is compiled by Cython. A thin __init__.py re-exports the public
names so all existing import sites continue to work unchanged.

Cython compatibility adjustments:
- Remove `from __future__ import annotations` (unsupported by Cython)
- Remove TYPE_CHECKING guard; quote annotations that reference Stream
  (circular import), forward-reference GraphBuilder/Graph, or use
  X | None union syntax
- Update _graphdef.pyx lazy imports to point directly at _graph_builder

No build_hooks.py changes needed — the build system auto-discovers .pyx
files via glob.

Ref: #1076
Made-with: Cursor

Author: Andy-Jost