Builds a DERIVED baseline from a generator baseline plus a plugin.
mix capstone.baseline.compose web:web is no longer generated. It is :api plus the web layer, so its tree is
produced rather than downloaded — which is what makes the Svelte layer's
deletions cease to exist rather than need representing.
Two steps: RENAME the source tree from its own names to the target's, then
APPLY the plugin. The rename is Capstone.Template.capture/2 followed by
render/2 — two functions that already exist and are tested — rather than
new machinery, and it runs before apply because a plugin is captured
against the source's names and resolves APP against the target's.
Separate from mix capstone.baseline.record on purpose. Record is pure over
directories and shells out to nothing, which is what lets the drift check run
offline on every commit; this task writes the tree, record hashes it.