Add INI filetype support

[0xTaoZ]

Fixes #49.

This adds first-class INI support using Python's standard configparser module. INI files are represented as section dictionaries with string option values, which lets Graphtage parse, diff, and format .ini files without adding a new dependency.

Changes:

• add an ini filetype with parser and formatter support
• register .ini MIME detection for CLI use
• include INI in the package filetype registry
• add randomized INI round-trip formatting coverage

Local validation:

• .venv/bin/python -m pytest test/test_formatting.py -q
• .venv/bin/python -m ruff check graphtage/ini.py
• .venv/bin/python -m py_compile graphtage/ini.py graphtage/__init__.py graphtage/__main__.py test/test_formatting.py
• git diff --check
• manual CLI smoke test diffing two .ini files

Note: full changed-file Ruff checking reports existing lint issues in files that already predate this change, so I scoped Ruff to the new module and used targeted tests for the touched behavior.