Collapse every ICU plural / select / selectordinal expression in
message to its other branch, recursively, and strip type/format
suffixes from typed placeholders ({amount, number, currency} →
{amount}). Literal text, simple {placeholder} tokens, and quoted
runs are preserved verbatim. This is the flat-json lowering rule —
see dialect/spec/flat-json.md.
True if message contains any plural, select, or selectordinal
expression at any nesting depth — i.e. anything flat-json would
collapse. Used to count lossy events for the sync hint.