21. Implement the transferable part, not the artefact

Context

Kircher's Arca musarithmica sets text to four-part music. A library of writing procedures has no representation for a four-part setting, no pitch, no rhythm, and no business acquiring them. On a literal reading the Arca does not belong here at all.

But the machine underneath is not musical. It measures a phrase, looks up the tablet indexed by that measure, and draws a column. The music is what Kircher put in the columns.

Decision

Implement the indexing and leave the columns opaque. arca_musarithmica counts a phrase's syllables, finds the tablet for that length, and verifies the chosen pattern is one that tablet offers. It never reads a pattern.

Put Kircher's pitch numbers in the table and you get his machine. Put stress patterns or rhyme schemes in and the same bones carry those — which a test asserts, because a claim about pluggability that nothing exercises is decoration.

Consequences

The catalogue row says exactly this, so nobody expects music from it. The distinction it draws is the same one sestina and eodermdrome draw: the entry names the form, the definition names what the checker verifies, and the two are not allowed to drift apart.

No tablet ships. Kircher's own runs to many pages of Musurgia Universalis book VIII and has not been transcribed into anything this project could verify — the same arrangement as the corpus and the pasigraphic vocabulary, and for the same reason.

A phrase whose length the tablet does not cover is unsettable, and gets its own error rather than a malformed-table one. Kircher's rods run to particular lengths; a text that exceeds them cannot be set by that box, and that is a fact about the box, not a fault in it.

The chapter also closed a gap in the invariant suite. satisfied == (score == 1.0) had been asserted from the beginning, but nothing asserted that a satisfied report carries no violations — so a violation the score never counted could produce a report saying the text passed and then listing what was wrong with it. The Arca's tone check did exactly that before it was fixed.