Rewrite egg package with Cython for performance optimization

[Copilot]

Converts the egg package from pure Python to Cython for performance optimization while preserving the public API and external interfaces.

Changes

• Core implementation: Converted __init__.py to __init__.pyx with C extension types

  • UnionFind, ENode, EGraph now use cdef class for C-level performance
  • Public methods use cpdef for Python/C interface
  • Internal methods and fields use cdef for C-only execution (no Python overhead)
  • Preserved Python's default hashing for ENode.children tuple

• Build system: Updated to support Cython compilation

  • Added Cython ~3.0.11 to build dependencies in pyproject.toml
  • Created setup.py with Cython extension configuration
  • Added .gitignore for Cython-generated C/C++ artifacts

• API surface: Maintained public API compatibility

  • Only functions without underscore prefix remain exposed
  • Member fields properly hidden via cdef declarations
  • apyds.Term integration unchanged (external interface)

All 20 existing tests pass without modification. The implementation compiles to a native C extension (~895KB .so file).

# Usage remains identical
from apyds_egg import EGraph
import apyds

eg = EGraph()
a = eg.add(apyds.Term("a"))
b = eg.add(apyds.Term("b"))
eg.merge(a, b)
eg.rebuild()

Original prompt

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This section details on the original issue you should resolve

<issue_title>使用cython改写/egg中的库</issue_title>
<issue_description>尽可能的优化，除了apyds的Term这个外部部分。

几个类型的member field其实并不是外部接口，只有那几个没有下划线开头的函数是外部接口，其他并不需要暴露。

enode的tuple的那个children实际上内容都是ClassId，只是要用Python默认的hash，所以设置成tuple的，写成cython时可能需要注意。

注意build system需要更新以支持cython。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 使用cython改写/egg中的库 #164

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