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

AGENTS.md

@@ -43,7 +43,9 @@ the spec_; do not port legacy code.
   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.
+  `/app/`, leaving the landing uncontrolled and always-fresh. **`/learn/` is a second such
+  entry** (`src/learn/`) — the markdown-authored deep-dive section, under the same rules
+  (see architecture 11).
 
 See [`docs/architecture/`](docs/architecture/00-overview.md) for the patterns behind each
 layer (state, persistence, modals, routing, rendering, inference, relationships) and
@@ -57,9 +59,11 @@ external repo is needed to work from them.
 ```
 index.html           # Landing entry (served at /)
 app/index.html       # App entry (served at /app/)
+learn/index.html     # Learning-section entry (served at /learn/)
 src/
 ├── main.tsx         # App bootstrap (font wiring, startup, render)
 ├── landing/         # Marketing landing at / — standalone page; reuses core + chart-renderer
+├── learn/           # /learn/ deep-dive — markdown lessons + the SpecProgression engine (arch 11)
 ├── core/            # Portable spec engine (no browser/React/Monaco)
 ├── app/
 │   ├── components/  # React UI (CSS Modules co-located)
@@ -71,7 +75,7 @@ src/
 styles/              # Global CSS (tokens, base)
 docs/
 ├── spec/            # Authoritative behavioral specification (00–10) — the WHAT
-├── architecture/    # Architecture playbook (00–10) — the HOW (self-contained)
+├── architecture/    # Architecture playbook (00–11) — the HOW (self-contained)
 │   └── visual-specimen.html  # Standalone token sandbox + reusable-primitive catalog
 ├── exploration/     # Point-in-time records (research, reviews, scope memos) — not maintained
 ├── IMPLEMENTATION-PLAN.md    # Milestone sequence (M0–M6)
@@ -151,6 +155,13 @@ High coverage on `src/core/` (parsing, detection, profiling, reference resolutio
 transforms, import normalization). Lighter on components. Extract testable logic out of
 components into core/stores where practical.
 
+Don't test static presentational components — copy, markup, and links with no logic behind
+them. Content assertions ("renders the heading X", "this link exists") are change-detectors:
+they break on intentional copy edits and catch no real bug. Test the logic a component
+carries — platform branches, state transitions, config-path writes, render serialization —
+not the strings it renders; if that logic is worth guarding, lift it into core/stores and
+test it there.
+
 Component tests (happy-dom) share a harness shape: `createRoot` + `act` with
 `IS_REACT_ACT_ENVIRONMENT = true` set at module level, stores reset in `beforeEach`, and
 `vi.mock('../services/chart-renderer', …)` for anything that embeds a chart (vega-embed is