add format engine packages: jovial_svg, markdown, html_core (D-058)

Three format engines adopted per the new D-058 rule (format
engines clear the dependency bar; UI chrome stays in-house):

- jovial_svg 1.1.26 (BSD-3) — SVG renderer via CustomPaint
- markdown 7.2.2 (BSD-3) — GFM parser; renderer is ours
- flutter_widget_from_html_core 0.17.2 (MIT) — HTML renderer

Welcome screen now loads logo.svg via jovial_svg instead of a
raster PNG. POLICY.md updated with format engine rule. All three
documented in licenses.yaml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -41,6 +41,12 @@ dependencies:
 
   ffi: 2.1.3
 
+  jovial_svg: 1.1.26
+
+  markdown: 7.2.2
+
+  flutter_widget_from_html_core: 0.17.2
+
 dev_dependencies:
   flutter_test:
     sdk: flutter