Preserve escaped newline text in formatted containers

[FanWu-ai]

Printing a configuration dictionary such as {'path': r'C:\new\report.csv'} currently inserts a newline into the displayed path. The generic argumentToString() replaces every backslash-n substring in pretty-printed output, including one that follows an already escaped backslash.

Consume escaped backslash pairs before converting a remaining newline escape. This preserves literal backslashes inside containers, bytes/bytearrays, and custom representations while retaining the existing rendering of real newlines and pretty-print wrapping. Top-level string dispatch is unchanged.

Five regression tests cover public ic() output, nested containers, odd/even backslash counts, bytes, custom repr, quotes and CRLF. All five fail against the original implementation (11 failing assertions/subtests).

Validation on Linux / CPython 3.12.14:

• Original full unittest suite: 65 passed, no skips.
• Final full unittest suite: 70 passed, no skips.
• Project-pinned mypy 1.7.1: all five source files passed.
• Source distribution and wheel builds passed.
• 6,320 additional escape/newline boundary checks and git diff --check passed.

Related to the path-display reports in #130/#206. I checked #228: it changes the registered string formatter, whereas this fix concerns the generic formatter used for containers and other values. This does not claim to resolve the broader output-policy questions in #220.

AI assistance was used to prepare the implementation and regression tests. The hosted Python/OS matrix has not been run locally.