Two-step decode in decodeLiteralEscapes:
- Step 1: regex anchored to high-surrogate range (U+D800–U+DBFF) so a
preceding BMP escape (e.g. \u0041) cannot consume the high-surrogate
half of a valid pair like \uD83D\uDE00 (😀), leaving \uDE00 lone.
- Step 2: decode remaining BMP codepoints; preserve lone surrogates as
six-character literals instead of corrupting them to U+FFFD in the
outbound IRC UTF-8 stream.
