Per-chart export: copy/download spec and PNG/SVG from the preview header

docs/spec/04-live-preview.md

@@ -36,6 +36,10 @@ Behavior of the selected mode:
 - The selected mode persists across sessions, stored in _Settings_ as `previewFitMode`.
 - The default is the natural Original mode.
 
+## Export control
+
+The preview pane header also carries a per-chart **Export** control — a disclosure for copying or downloading the current chart's spec, or downloading its rendered image (PNG/SVG). It exports what the preview shows. The behavior is specified in _Import & Export → Per-chart export_; it lives in this header because the image formats are produced from the live rendered view.
+
 ## Rendering Contract
 
 Before the chart is drawn, the spec shown in the editor is transformed into the spec actually rendered. Two deterministic transforms are applied in order. They are specified here because reproducing them faithfully is what makes references and fit modes behave correctly; the result is observable as the rendered chart.

docs/spec/08-import-export.md

@@ -1,6 +1,8 @@
 # 08 · Import & Export
 
-Astrolabe lets a user back up or transfer their entire workspace as a single JSON file, and bring data back in by importing such a file. Both actions are triggered from header controls labelled **Import** and **Export**. Import always merges with existing data; it never replaces what is already stored.
+Astrolabe lets a user back up or transfer their entire workspace as a single JSON file, and bring data back in by importing such a file. These two whole-workspace actions are triggered from header controls labelled **Import** and **Export**. Import always merges with existing data; it never replaces what is already stored.
+
+Separately, a single chart can be exported on its own — its spec or its rendered image — from the Live Preview pane (see _Per-chart export_ below). That is distinct from the workspace Export: it gets _one_ chart out, not a backup of the library.
 
 ## Export
 
@@ -35,6 +37,29 @@ The downloaded file is a single JSON object: an envelope with a format `version`
 - `exportedBy` — fixed identifier `"Astrolabe"`.
 - `snippets` / `datasets` — arrays of complete records as defined in _Data Model_, each including its record `version` field. (This is the per-record schema version, not the envelope `version` above.)
 
+## Per-chart export
+
+A single chart can be exported on its own, separately from the whole-workspace Export above. The affordance is an **Export** control in the **Live Preview pane header** — placed there because exporting an image needs the chart that is currently rendered, and "export this chart" reads naturally beside the chart you are looking at. It is a disclosure that reveals four actions in two groups:
+
+**Spec** (always available when a snippet is open):
+
+- **Copy spec** — copies the currently-shown spec (draft or published, matching the editor's view) to the clipboard as JSON. Confirmed by a success toast, since a clipboard write is otherwise invisible.
+- **Download JSON** — downloads the currently-shown spec as a `.vl.json` file.
+- **Referenced data** (shown only when the spec references one or more saved datasets) — a choice between **Inline** (default) and **Keep refs**. _Inline_ replaces each `{ data: { name } }` reference with the dataset's actual values so the exported spec renders standalone (outside Astrolabe), leaving the authored sizing untouched; _Keep refs_ exports the reference as written (which only resolves inside Astrolabe). It applies to both spec actions. If inlining is requested but a referenced dataset is missing from the library, the export is declined with a clear error.
+
+**Image** (available only when a chart is currently rendered — the actions are disabled, with an explanatory line, while the preview is empty or showing an error):
+
+- **Download PNG** — a rasterized image of the chart as shown.
+- **Download SVG** — a vector image of the chart as shown.
+- **Resolution** (PNG) — `1×` / `2×` / `3×`, default `1×`. These are multipliers **of the display's pixel density**, so `1×` already matches on-screen crispness on a high-DPI (Retina) display; higher values produce larger images for print or zoom. (SVG is resolution-independent and ignores this.)
+- **Background** — `Theme` (default) / `White` / `None`. The chart itself renders on a transparent background (so on screen it shows the pane colour); export therefore fills it: _Theme_ matches the active theme's background, _White_ is always white, _None_ keeps it transparent. Applies to both PNG and SVG.
+
+Every format exports _what is on screen_: the same spec the editor shows and the same chart the preview renders (current fit mode included). With a _Theme_ background the exported image reflects the active light/dark theme.
+
+- **Filename**: derived from the snippet's name, made filesystem-safe (whitespace and illegal characters normalized; letters of any script preserved), with the format as the extension — e.g. `sales-by-region.png`, `sales-by-region.vl.json`. A name with nothing usable falls back to `chart`. No date or `astrolabe-` prefix (unlike the workspace export) — the user is exporting one named chart and wants its name on the file.
+- **Feedback**: a success toast naming the saved file (or confirming the copy); a clear error if the clipboard is blocked or the chart is not ready to rasterize.
+- **Availability**: the Export control is disabled when no snippet is open. The image actions additionally require a live rendered chart.
+
 ## Import
 
 Import lets the user pick a JSON file from their device; its contents are normalized, merged into the current workspace, and saved.