Escape content appended to a String via __add__

[Master-Norna]

Summary

String.__add__ spliced the appended text into the raw literal without escaping it, so appending a quote or backslash to a basic string produced a literal that is not valid TOML - including to this library's own parser:

i = item("foo")
i += '"'
i.as_string()   # before: '"foo""'  (unparseable)
                # after:  '"foo\""'

Escape the appended plain text with the string type's escape sequences (a String operand keeps contributing its already-escaped raw form), mirroring what from_raw already does. This is the same class of bug as #231, which fixed the literal round-trip but not __add__.

Agent Drafting Metadata

• Agent: Claude (Anthropic)
• Model: Claude Opus
• Notes: Agent assisted with investigation and drafting; the fix, tests and verification were reviewed by the submitter. Full suite passes (1059 passed).