Replace the first-run placeholder seed with an onboarding canvas

docs/architecture/10-interaction-and-feedback.md

@@ -265,15 +265,38 @@ default Modified/desc); search is **not** persisted (it's a transient view narro
 _(Consulted via /council → WAI-ARIA APG disclosure + menu-button, NN/g #6. This bullet is the
 contract; cite it, not the source.)_
 
-**Resolved — the two library empty states (Carbon's two types).** The snippet list owes
-**two distinct empty states** (Carbon empty-state types), chosen by cause and never conflated:
-(a) a **genuinely empty library** (first run) is **positive and concise and guides to the
-Create action** — title "No snippets yet" + a one-line nudge to create one; (b) **no search
-matches** says "No snippets match your search" + a hint to try a different term. Both are
-plain-language and minimal (NN/g aesthetic-and-minimalist; §3 empty ≠ error). The library
-derives **which** to show from whether a query is active, not from the raw count alone.
-_(Consulted via /council → Carbon empty-state, NN/g. This bullet is the contract; cite it,
-not the source.)_
+**Resolved — the empty-library onboarding canvas (and the list's single empty state).** The
+snippet list owes **one empty state**: **no search matches** — "No snippets match your search"
+
+- a hint to try a different term. There is **no separate "empty library" list state**, because
+  a genuinely empty library never shows the list at all: rather than seeding a placeholder
+  snippet (the old behavior), the workspace replaces the **entire pane chrome — toggle strip,
+  library list, editor, and preview — with a full-width onboarding canvas** (spec §02 →
+  First-Run & Empty Workspace): with no snippets, the library controls and pane toggles have
+  nothing to act on. (So the list's own empty copy is reached only mid-search, never on a cold
+  start.) The canvas is a welcome, a primary "Create your first
+  snippet", and a gallery of example snippets; leaving it (creating the first snippet) lays the
+  panes out at a default **25·25·50** split via `PanesStore.applyOnboardingSplit` so the first
+  chart opens with a generous preview. Each card
+  **renders live through the shared `chart-renderer`** (no parallel embed path; each card owns
+  its `RenderHandle` and finalizes on unmount — the per-card nodes are independent, so they
+  don't touch `LivePreview`'s single-host serialization), and adds as an ordinary snippet.
+  Empty stays calm and positive, never an error (NN/g aesthetic-and-minimalist; §3 empty ≠
+  error). Council resolutions for the canvas: (1) **each card preview is decorative** —
+  `aria-hidden`, skipped by screen readers (Carbon empty-states a11y / WCAG decorative
+  images); the card **name + one-line description + a uniquely-labelled `Add` button** ("Add
+  Bar chart", APG button) carry the meaning, so AT users reach no dead end. (2) The canvas is
+  the **single empty surface** — because it replaces the library list outright, there is no
+  competing "No snippets yet" status elsewhere to keep in sync. Its heading **owns the app
+  identity** ("Welcome to Astrolabe") and it carries the only Create nudge, so the empty-state
+  message lives in exactly one place (Carbon "keep words to a minimum"; no duplication).
+  (3) The **primary action dominates** (accent "Create your first snippet" first; the example
+  gallery is framed as a secondary "Or start from an example") — Carbon sanctions starter
+  content as an in-depth first-use empty state only when one action stays primary. (4)
+  **Domain vocabulary stands**: "Vega-Lite", "JSON", "snippet" are kept despite Carbon's
+  avoid-jargon rule, because SOUL #2 (Vega-Lite Native) makes them the user's real language —
+  a deliberate divergence. _(Consulted via /council → Carbon empty-state + content, GOV.UK
+  headings, WAI-ARIA APG button, NN/g. This bullet is the contract; cite it, not the source.)_
 
 **Resolved — one live region per shared message.** When the same error feeds two surfaces
 (the §1 "one producer, two subscribers" case — render errors via `PreviewStore`), exactly