Learn: a /learn/ deep-dive section — markdown lessons as before/after spec progressions

docs/architecture/11-learning-section.md

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+# 11 — The Learning Section (`/learn/`)
+
+An interactive deep-dive into Vega-Lite, served at `/learn/`: a marketing surface separate
+from the app, where each lesson walks a spec from an 80%-naive version to a polished one to
+teach the grammar's dormant power and funnel readers into the app.
+
+## A marketing surface, like the landing
+
+`/learn/` is a third Vite entry (`learn/index.html` → `src/learn/`) alongside the landing
+(`/`) and the app (`/app/`), under the same marketing-surface rules:
+
+- Reuses **`src/core` and the landing's `LandingChart`** only — never stores, modals,
+  orchestration, or app components. Vega is lazy-loaded through `LandingChart`, so the entry
+  stays light.
+- **Out of PWA scope.** The service worker is scoped to `/app/`, so the learning pages stay
+  uncontrolled, always-fresh, and indexable — the point for organic-reach content.
+
+## Lessons are markdown; the renderer is general
+
+A lesson is a `.md` file in `src/learn/lessons/`, discovered with `import.meta.glob` — so
+**adding a lesson is dropping a file**, with no registry to edit. A lesson is a _free-form
+document of ordered blocks_, not a fixed template:
+
+| Authored as                                          | Block         | Rendered by                 |
+| ---------------------------------------------------- | ------------- | --------------------------- |
+| plain markdown                                       | `prose`       | `Markdown`                  |
+| `:::progression` wrapping `##` stages + fenced specs | `progression` | `SpecProgression`           |
+| a bare fenced `vega-lite` block                      | `chart`       | `LandingChart`              |
+| `:::name … :::`                                      | `callout`     | `Markdown` (styled by name) |
+
+Inside a `:::progression`, each `##` heading is a stage: heading → tab label, prose → note,
+the following fenced `vega-lite` block → spec. Inline data repeats per fenced block — there
+is no shared-data construct.
+
+## The pipeline
+
+`lessons/*.md` → `import.meta.glob` (in `LearnPage`) → `parseLesson` (`core/lesson-parse`) →
+`LessonBlock[]` → `LearnPage` block dispatch → `Markdown` | `SpecProgression` | `LandingChart`.
+`SpecProgression` renders each stage's spec with `formatSpec` (`core/json-format`) and
+highlights what the stage changed with `changedLines` (`core/spec-diff`, an LCS line-diff).
+
+## Rules
+
+- **The engine consumes plain data, so the authoring surface is swappable.**
+  `SpecProgression`, `spec-diff`, and `formatSpec` take parsed blocks and specs and know
+  nothing about markdown — the parser is the only thing coupled to the `.md` format. The
+  authoring format can change without touching the widget.
+- **Parsing, diff, and formatting are pure and live in `core`** (tested hardest); rendering
+  and the markdown library live in `src/learn`. **Core stays dependency-free** — `marked` is
+  imported only by `src/learn/Markdown`.
+- **`dangerouslySetInnerHTML` renders first-party lesson files** (repo content authored by
+  us), never user input — not an XSS surface.
+- Lesson specs are fenced JSON parsed with `JSON.parse`; the source pane re-formats them with
+  `formatSpec` so the shown JSON matches the editor's house style.