pdf: text that names no colour comes out black, not the palette

A QTextDocument takes its default text colour from the application
palette, and markdown names no colour at all: in a window running a dark
palette -- which is what a phone in dark mode hands Qt -- every plain
paragraph was laid out #eeeeee on a white page. Printed, that is a blank
document; on screen it reads as "the font is oddly faint". The first
PDFs off the phone were exactly that.

- PdfTools: paperDocument() sets the default stylesheet (body colour
  #000000) before setHtml, and writePdf() lays that out. A colour the
  document does name still wins, so an .html handed to the tool keeps its
  own -- checked with a #cc0000 document under both palettes.
- --uitest: draws that document on a white image under a dark palette and
  looks for ink (238 before the fix, 0 after).

Verified on the NX789J: md_to_pdf on the device now writes CID TrueType
NotoSansCJKHK-Regular/-Bold text whose page renders at min=0.

app/src/core/ToolProvider.h

@@ -19,6 +19,9 @@
 /* The engine wrapper of the front end (core/AgentCore.h). Only a pointer
  * travels through this header, so the class stays incomplete here. */
 class AgentCore;
+/* Same for the document md_to_pdf lays out (PdfTools.cpp): the caller owns
+ * it, and the test in ui/UiTest.cpp inspects how it draws. */
+class QTextDocument;
 
 namespace app {
 
@@ -48,6 +51,10 @@ int registerPlatformTools(AgentCore *core);
  * (they are Qt, not a platform API): today md_to_pdf, owned by "app:qt".
  * Called next to registerPlatformTools(). */
 int registerQtTools(AgentCore *core);
+/* The document md_to_pdf lays out from `html`: the page's own defaults
+ * applied (text with no colour of its own is black, not the window's palette
+ * colour). The caller owns the result. */
+QTextDocument *paperDocument(const QString &html);
 /* Drop them again (they belong to the owners "app:*"). */
 void unregisterPlatformTools(AgentCore *core);