Landing: add marketing page at /, move the app to /app/

Multi-page Vite build: index.html serves a new standalone landing
(src/landing), app/index.html serves the app at /app/. The PWA service
worker and manifest are scoped to /app/, so the landing stays
uncontrolled and always-fresh; hash view-state routing is unchanged.

The landing reuses src/core and the chart-renderer service only (never
stores/orchestration/modals/components) and lazy-loads Vega. Its demos
drive the real core: a hero snippet switcher over CHART_EXAMPLES, an
interactive Chart Builder over a new profiled core fixture
(sample-dataset), and a Theme Builder gallery with three from-scratch
custom themes plus built-in presets.

AGENTS.md

@@ -38,6 +38,12 @@ the spec_; do not port legacy code.
 - **Modals** go through a registry + coordinator + shell, not ad-hoc rendering.
 - **CSS Modules + design tokens** (`styles/tokens.css`); themes flip `[data-theme]`.
 - **No shared library with Syto** — patterns are adapted, never imported.
+- **The app is served at `/app/`; `/` is a standalone marketing landing** (`src/landing/`).
+  Multi-page Vite build — `index.html` → the landing, `app/index.html` → the app (hash
+  view-state routing is unchanged by the base path). The landing reuses `src/core` and the
+  `chart-renderer` service only — never stores, orchestration, modals, or components — and
+  lazy-loads Vega, so `/` stays light. The PWA service worker and manifest are scoped to
+  `/app/`, leaving the landing uncontrolled and always-fresh.
 
 See [`docs/architecture/`](docs/architecture/00-overview.md) for the patterns behind each
 layer (state, persistence, modals, routing, rendering, inference, relationships) and
@@ -49,8 +55,11 @@ external repo is needed to work from them.
 ## Directory Structure
 
 ```
+index.html           # Landing entry (served at /)
+app/index.html       # App entry (served at /app/)
 src/
-├── main.tsx         # App entry (font wiring, startup, render)
+├── main.tsx         # App bootstrap (font wiring, startup, render)
+├── landing/         # Marketing landing at / — standalone page; reuses core + chart-renderer
 ├── core/            # Portable spec engine (no browser/React/Monaco)
 ├── app/
 │   ├── components/  # React UI (CSS Modules co-located)