const MASK_CHAR: char = '\u{E000}';Expand description
Private-use code point used to stand in for an Aozora trigger
character that lives inside a fenced code block. Distinct from
aozora_pipeline::INLINE_SENTINEL (U+E001) and the three block
sentinels (U+E002..U+E004), so the masking pass cannot collide
with the lexer’s own sentinels.