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

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.