9. Diacritic folding is a parameter, not a policy

Context

Whether ä counts as a has no single right answer. A German lipogrammatist writing without "a" must decide whether Mädchen is permitted, and Perec's translators faced exactly that question; the French edition had already settled that é, è and ê count as e. The same question reaches English through borrowings like café.

Meanwhile two kinds of procedure must never fold. snowball counts the letters of a word as written. prisoners_constraint asks whether a glyph rises above the x-height, and folding is precisely what destroys the information it needs.

Decision

fold_diacritics is a per-call parameter defaulting to true, carried on a shared DiacriticParams base model and inherited by the nine procedures that compare letters. letter_spans takes a matching fold argument. Procedures that count word length or read glyph shapes do not inherit the model, so the parameter cannot be passed to them at all.

Consequences

The choice reaches params_schema() and therefore the JSON data contract, and works as --param fold_diacritics=false from the command line, without any further plumbing. Every letter-comparing procedure now has two code paths, and both need fixtures.

The exclusion is enforced by type rather than by convention: prisoners_constraint has no fold_diacritics field, so no caller can ask it to fold, and a future procedure that inherits the wrong base will be caught by the parameter appearing where it should not.