Ask what a single note's export should contain, and fix drawings never being in one

Two questions instead of one, because the answers depend on each other:
Markdown holds text only, a PNG holds the drawing only, PDF holds both.
Listing every format flat would have offered combinations that cannot
exist. A note without a drawing skips the first step rather than showing
it mostly greyed out.

While testing that, the drawings turned out never to have made it into
any export at all — not the ZIP, not the PDF. strokeDocOf() *builds* a
stroke document from a bare array, and it was being handed one that was
already a document, so the strokes ended up nested one level too deep and
every length check read undefined. Nothing complained; the export simply
came out without the drawing. It went unnoticed because the notes the
earlier suite exported had none.

Now uses docStrokes(), and the suite draws on a note before exporting it,
checks the PNG's magic bytes, and checks it is 159 KB rather than an empty
white page — a check on the file merely existing would have passed against
the broken version too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

readme.md

@@ -332,7 +332,7 @@ docker compose -f docker-compose.dev.yml up -d --build   # must be running
 | `search-attachments` | 12 | body-only matches, ranking, umlauts; images as blob URLs, not inline bytes |
 | `history` | 10 | a large deletion is snapshotted, restoring works, restoring is itself undoable |
 | `drawing` | 7 | strokes survive the poller, and match across four viewport widths |
-| `export` | 12 | the downloaded bytes are real Markdown and a real ZIP, unpacked and read back |
+| `export` | 21 | the downloaded bytes are real Markdown, ZIP and PNG, unpacked and read back |
 
 Each suite creates a throwaway account through the admin API and deletes it
 afterwards, so runs leave the database as they found it. They need an admin to
@@ -485,6 +485,19 @@ Notes should not be hostage to this program, so everything can leave it — from
 single note up to every note in the account, reachable from the note's toolbar,
 a notebook's context menu, and Account → *Alle Notizen exportieren*.
 
+A single note is asked two questions rather than one, because the answers are
+not independent — Markdown holds text only, a PNG holds the drawing only, and
+PDF is the one format that holds both:
+
+| Was soll mit? | In welcher Form? |
+| --- | --- |
+| Notiz mit Zeichnung | PDF *(Markdown puts the drawing in a PNG beside it)* |
+| Nur den Notiztext | PDF · Markdown |
+| Nur die Zeichnung | PNG · PDF |
+
+A note without a drawing skips the first question instead of showing it with two
+thirds greyed out.
+
 **Markdown** keeps the notes editable elsewhere. Headings, lists, quotes, code,
 tables and task state all have equivalents and are carried over; a link to
 another exported note becomes a relative link to its file, and a link to a note