What do these changes do

Adds comprehensive type hints to 3 of 4 foundation submodules per stage 1 goal. Establishes mypy configuration and type checking infrastructure for the codebase.

Completed modules (23 files):

• pythainlp.tools - 4 files, passes mypy strict
• pythainlp.cli - 7 files, passes mypy strict
• pythainlp.corpus - 12 files, all exported functions typed

Remaining: pythainlp.tokenize (24 files, 3360 lines) - deferred due to complexity

Infrastructure:

• Mypy configuration in pyproject.toml
• Python 3.9 compatible types (Union not |, native types, TYPE_CHECKING blocks)
• Strict checking enabled for completed foundation modules
• Optional dependency imports ignored

What was wrong

Foundation submodules lacked type hints, preventing:

• Static type checking with mypy
• IDE type inference and autocompletion
• Runtime type inspection by tools
• Path to typed package status

How this fixes it

Type hint additions:

• Function signatures: return types and parameter types
• Class attributes: proper type annotations
• Module-level variables: explicit types for collections

Patterns applied:

# Before
def get_corpus(filename, comments=True):
    return frozenset(...)

# After
def get_corpus(filename: str, comments: bool = True) -> frozenset[str]:
    return frozenset(...)

TYPE_CHECKING usage:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from collections.abc import Sequence

def func(argv: Sequence[str]) -> None: ...

Standards enforced:

• Union[A, B] for Python 3.9 (not A | B)
• Native generics: list[str] not List[str]
• Optional[T] explicit
• Specific generic parameters: frozenset[str] not frozenset

Your checklist for this pull request

• Passed code styles and structures
• Passed code linting checks and unit test

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