Added iMessage Support for MacOS

[Gear-I]

Adds four new artifacts parsing ~/Library/Messages/chat.db on macOS: Messages, Attachments, Chat Threads, and Deletion Tombstones.

Validated against a real chat.db from Josh Hickman's public macOS Big Sur test image (thisisdfir persona): 25 messages, 5 attachments, 1 chat thread, 1 handle — see sample_data on each artifact.

Notes:

• attributedBody (NSKeyedArchiver rich-text blob) is not decoded; rows where text is empty but attributedBody is present are flagged in the Body Source column rather than shown as empty.
• message.date/date_read/date_delivered use Mac Absolute Time in nanoseconds; attachment.created_date/start_date use seconds — confirmed against real data, documented in the module docstring."

Code was co-authored by Claude Sonnet 5