feat(prototype): logic branch produces a shareable HTML demo

Reshape the logic branch from a terminal app into a single
self-contained HTML file a non-developer can drive: a labelled
state panel, free-play buttons, and tabbed guided walkthroughs
(scenarios) with the ordered buttons to press underneath each.
The portable pure-logic module still lifts into the real code;
the HTML shell is the throwaway primary source.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/engineering/prototype.md

@@ -26,10 +26,10 @@ Reach for it when you have a design question that's hard to settle on paper —
 
 The question decides the shape, and there are two shapes:
 
-- **"Does this logic / state model feel right?"** — a tiny interactive terminal app that pushes the state machine through the awkward cases, printing the full state after every action so you can watch what changes.
+- **"Does this logic / state model feel right?"** — a single shareable HTML file, free-play buttons plus tabbed guided walkthroughs, that pushes the state machine through the awkward cases and shows the full state after every click, so anyone — including a non-developer — can watch what changes.
 - **"What should this look like?"** — several radically different UI variations on one route, switchable from a floating bar, so you compare real renders instead of imagining them.
 
-Picking the wrong branch wastes the whole prototype, so the question comes first. Both branches keep state in memory, run from one command, and surface the full state on every step.
+Picking the wrong branch wastes the whole prototype, so the question comes first. Both branches keep state in memory, are trivial to run, and surface the full state on every step.
 
 ## Keep the prototype as a primary source