Avoid leaking decoration state in remove_format

[ShipItAndPray]

Summary

• stop Formatter.remove_format() from mutating the shared _decorated flag while stripping styles
• use an operation-local suppression flag so decoration stays disabled only for that formatting call
• add a deterministic threaded regression covering shared formatter use between a StreamOutput and SectionOutput

Scope

This fixes the leaked undecorated state described in #423. It does not claim to make the formatter generally thread-safe.

Testing

• .venv/bin/pytest tests/io/outputs/test_section_output.py -k shared_decoration_state -q
• .venv/bin/pytest tests/formatters/test_formatter.py tests/io/outputs/test_section_output.py -q

Closes #423.