Function is_sentinel_char 

pub(crate) const fn is_sentinel_char(ch: char) -> bool

True iff ch is one of the four PUA sentinel codepoints U+E001..=U+E004.

Implemented as a single subtract-and-compare. The optimiser would likely fold the equivalent matches! chain into the same code, but writing it once explicitly keeps the hot path obvious to readers and lets us const-eval it where needed.