v0.40.0 — The first composed rune

v0.38.0 and v0.39.0 built the prerequisites: the ownership marker, declarative registration, slot labelling and tree-order preprocess. This milestone uses them to ship the mechanism they were for. A rune defined as a Markdoc template placing other runes (SPEC-145), proved on two real runes from the storytelling plugin, with the packaging groundwork composed runes will need to be delivered (SPEC-153).

The decision pass that settled SPEC-145's blocking questions is done (D2a, D10a–c, D25, D26). What remains open in the spec does not block the two slices below.