make test no longer runs the frontend's production build (refs #118)

yarn build moves out of the stage that runs the frontend's unit tests,
now named frontend-test, into a frontend-build stage of its own. It
starts from frontend-test for its yarn packages and source, installs
the git that vite.config.js reads the commit with, and copies from the
lint and test phases as builder does, so it cannot run unless they
passed. The runtime stage copies dist/ from it. The Go compile, most of
make test's time, is unchanged and waits on the ruling asked for in
sneak/prompts#113; no build cache is added.

Model: opus-5-5

script/frontend-test

@@ -2,9 +2,10 @@
 # script/frontend-test: run the frontend test suite on the host: the
 # unit tests in test/unit/, through the test script in package.json,
 # then the production build, which fails on broken code. make test runs
-# the same in the frontend stage of Dockerfile. The tests print a dot
-# each; if any fails, they run again with every test listed, and the
-# script fails even if that run passes. NODE_OPTIONS chooses the
+# the same tests in the frontend-test stage of Dockerfile, and the image
+# build runs the production build in its frontend-build stage. The tests
+# print a dot each; if any fails, they run again with every test listed,
+# and the script fails even if that run passes. NODE_OPTIONS chooses the
 # reporter because yarn adds its arguments after the test files, where
 # node would take a reporter option for one more file.
 set -eu