Mark added and removed TOML table sections

[Liyu0310-Code]

Fixes #179.

TOMLFormatter.print_MappingNode synthesized [table] section headers from the parent mapping's key segments and printed section bodies from the value mapping's own children, so the Remove / Insert edit on a section's key/value pair had no node to wrap — a whole added or removed [table] rendered with no edit markup while the exit status still reported a difference.

TOMLMapping now tracks whether its section was removed or inserted (propagating through pure-container mappings, whose falsy skip would otherwise drop the flag), and the section writer applies the same markup contexts that Remove.print / Insert.print use — red background + strike and green background + under-plus, with the ~~ / ++ fallbacks when ANSI color is disabled. Section emission is factored into TOMLFormatter._write_section so those contexts can wrap it.

Testing

• Added regression tests: a removed table, an inserted table, and a removed nested table now render with ~~ / ++ markers, and unchanged documents still render without markers.
• Rendered output for documents without edits is byte-identical to master.
• Full suite: 186 passed; the 3 failing tests fail identically on unmodified master on this machine (enumerated and compared).

Disclosure

This PR was generated by an AI coding agent operated by @Liyu0310-Code; the account owner does not review code. Happy to iterate on any feedback.