Bring the tutorial up to the interface that exists, and check its code in CI

.github/workflows/ci.yml

@@ -58,6 +58,15 @@ jobs:
           python3 tools/make-icons.py
           git diff --exit-code -- resources/
 
+      # The tutorial tells a reader to type out whole files. If one stops
+      # parsing they find out by typing it in and getting a syntax error, with
+      # no way to tell whether they mistyped it or the page is wrong - the worst
+      # failure available to a document whose premise is "type this". The page
+      # spent two rebuilds describing an interface the code no longer had; prose
+      # going stale is hard to catch, code going stale is not.
+      - name: The tutorial's code still parses
+        run: python3 tools/check-tutorial.py
+
       # Every source file has to at least parse. `ghost <file>` reports a syntax
       # fault before it evaluates anything, so a file that only fails on a
       # `lumen:` import is fine - a file that fails to parse is not.

README.md

@@ -286,9 +286,7 @@ Lumen's audio API exposes no samples and no playhead).
 [docs/picotron.md](docs/picotron.md) is the current spec: every measurement the
 interface is built against, how they were taken, and which of them are exact.
 
-[docs/tutorial.html](docs/tutorial.html) builds the repository from an empty folder,
-explaining the reasoning as it goes — **but it documents the interface as it was before
-the Picotron rebuild**, and its chapters on painting and theming are now wrong. The
-structural half (the widget tree, the dock, input capture, commands, the keymap) and the
-Ghost papercut reference are still accurate. Rewriting it against the current design is
-outstanding work.
+[docs/tutorial.html](docs/tutorial.html) builds the repository from an empty folder, in
+order, explaining the reasoning as it goes. Open it in a browser. Its code blocks are
+checked in CI: every file the reader is told to type is parsed on every commit, because
+a tutorial whose code has quietly stopped working is worse than no tutorial.