Unknown charset labels (QQ Mail's RFC 1428 'unknown-8bit' placeholder,
misspelled names, garbage encoded-word charsets) raised LookupError from
bytes.decode — errors='replace' only guards decode errors, not a missing
codec — aborting the whole fetch batch. UIDs are marked seen before the
fetch, so the crash permanently dropped every message in the batch.
- _safe_decode(): alias table (unknown-8bit→utf-8, gb2312/gbk→gb18030,
ks_c_5601-1987→cp949, ...) then utf-8, then latin-1 last resort.
- _decode_header_value(): wraps decode_header() so a malformed RFC 2047
header degrades to the raw string instead of crashing.
- _extract_text_body(): all three decode sites now use _safe_decode.
Fixes#35901, fixes#55381, fixes#55383.
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