docs: animated explainer, linked from the README

README.md

@@ -136,6 +136,12 @@ plexus is the discovery layer that sits *above* an executor, not another executo
 
 <p align="center"><img src="docs/art/wiring-lane.svg" alt="Eight stages from manifest to verify, ending in still holds or drifted." width="100%"></p>
 
+## See it work, step by step
+
+The [animated explainer](https://harperz9.github.io/repo-explainers/plexus.html)
+walks through two small manifests wired into one edge, the unmet input and terminal output plexus names, a plan with its receipt, verify passing and then failing after a manifest changes, and the built-in ten-tool mesh. Every value on it is output from this repository. Its
+source is [docs/explainer/index.html](docs/explainer/index.html).
+
 ## The problem
 
 You wire up a set of tools. Each one produces artifacts and accepts inputs, but

docs/explainer/README.md

@@ -0,0 +1,13 @@
+# plexus explainer
+
+`index.html` is a single self-contained page that walks through how this
+repository works. It is published at
+<https://harperz9.github.io/repo-explainers/plexus.html>.
+
+Open `index.html` in a browser to read it from a checkout. It loads two
+typefaces from harperz9.github.io and falls back to system fonts offline.
+
+The commands, values and verdicts on the page came from running this repository
+at commit d889be9, and each step cites the file it describes. When the behaviour
+it shows changes, rerun the commands in the page's "Try it" section and update
+the values in the same pull request.