Replace the first-run placeholder seed with an onboarding canvas

docs/spec/02-snippet-library.md

@@ -10,8 +10,21 @@ The list shows every saved snippet and is always visible. A persistent "Create N
 - A "Create New Snippet" item is pinned at the top of the list; activating it creates and selects a new snippet (see _Snippet Operations_).
 - Selecting a snippet makes it the **active snippet**: it loads into the editor and preview, becomes highlighted in the list, and the URL updates to reflect the selected snippet so the state is shareable and survives a page reload (see _Application Shell & Navigation_).
 - Exactly one snippet is active at a time.
-- When no snippets match the current search, the list shows an empty-state message ("No snippets match your search", with a hint to try a different term); when there are genuinely no snippets, it shows "No snippets yet" with a nudge to create one.
-- On first run, when no snippets exist, the app seeds one sample bar-chart snippet so the user starts with a working example.
+- When no snippets match the current search, the list shows an empty-state message ("No snippets match your search", with a hint to try a different term). This is the list's only empty state: a genuinely empty library never shows the list at all (see next).
+- When the library is empty (first run, or after the last snippet is deleted), the workspace presents a full-width **onboarding canvas** in place of the panes — including the library list — rather than seeding placeholder content (see _First-Run & Empty Workspace_).
+
+## First-Run & Empty Workspace
+
+When the library is empty — on first run, or after the user deletes their last snippet — the app does **not** seed placeholder content. Instead the **onboarding canvas takes the full workspace**, replacing the pane chrome (the pane toggle strip, the library list, the editor, and the preview): with no snippets, the library's create/search/sort/storage controls and the pane toggles have nothing to act on, so the welcome gets the whole width. The user starts from a deliberate choice rather than dropped into the middle of an unfamiliar spec.
+
+- The canvas briefly identifies what Astrolabe is, then offers two ways to begin.
+- **Create your first snippet** — the primary action; starts a new snippet from the sample bar-chart template and opens it in the editor (identical to _Create New_ under _Snippet Operations_).
+- An **example gallery** of a few simple snippets showcasing distinct Vega-Lite capabilities (e.g. a bar chart, a time-series line, a scatter plot, a stacked area, a donut, a binned histogram). Each example shows a **live preview** of the chart and a one-line description.
+  - **Add** on an example creates it as an ordinary snippet and makes it active (opening it in the editor).
+  - **Add all** creates the whole set at once and makes one of them active.
+- Added examples are **ordinary snippets**: meaningfully named (not auto-generated timestamps), and thereafter editable, duplicable, and deletable like any other — they are the user's, not a special class (_own your data_).
+- Leaving the canvas by creating the first snippet(s) lays the workspace out at a sensible **default split** (library · editor · preview ≈ 25 · 25 · 50) with all three panes shown, so the first chart opens with a generous preview rather than the generic remembered widths.
+- The onboarding canvas is shown **only while the library is empty**; as soon as any snippet exists, the normal panes return. Re-emptying the library brings it back.
 
 ## List Item