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.lock

@@ -105,6 +105,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.7"
+  csslib:
+    dependency: transitive
+    description:
+      name: csslib
+      sha256: "09bad715f418841f976c77db72d5398dc1253c21fb9c0c7f0b0b985860b2d58e"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.2"
   equatable:
     dependency: transitive
     description:
@@ -160,6 +168,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_widget_from_html_core:
+    dependency: "direct main"
+    description:
+      name: flutter_widget_from_html_core
+      sha256: "7ff010b116f6abc16429923e616fbc727f3f65ef4cee12ffdb280aeecbc21e7f"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.17.2"
   frontend_server_client:
     dependency: transitive
     description:
@@ -181,6 +197,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.3"
+  html:
+    dependency: transitive
+    description:
+      name: html
+      sha256: "6d1264f2dffa1b1101c25a91dff0dc2daee4c18e87cd8538729773c073dbf602"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.15.6"
+  http:
+    dependency: transitive
+    description:
+      name: http
+      sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.0"
   http_multi_server:
     dependency: transitive
     description:
@@ -210,6 +242,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.5"
+  jovial_misc:
+    dependency: transitive
+    description:
+      name: jovial_misc
+      sha256: "4301011027d87b8b919cb862db84071a34448eadbb32cc8d40fe505424dfe69a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.2"
+  jovial_svg:
+    dependency: "direct main"
+    description:
+      name: jovial_svg
+      sha256: a3cab3d46dc8df80cf0792e5e4246cbe13287542d9a357007114b1057da2fe89
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.26"
   leak_tracker:
     dependency: transitive
     description:
@@ -250,6 +298,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.0"
+  markdown:
+    dependency: "direct main"
+    description:
+      name: markdown
+      sha256: ef2a1298144e3f985cc736b22e0ccdaf188b5b3970648f2d9dc13efd1d9df051
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.2.2"
   matcher:
     dependency: transitive
     description:
@@ -314,6 +370,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.9.1"
+  petitparser:
+    dependency: transitive
+    description:
+      name: petitparser
+      sha256: "91bd59303e9f769f108f8df05e371341b15d59e995e6806aefab827b58336675"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.2"
   platform:
     dependency: transitive
     description:
@@ -551,6 +615,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.2.1"
+  xml:
+    dependency: transitive
+    description:
+      name: xml
+      sha256: "971043b3a0d3da28727e40ed3e0b5d18b742fa5a68665cca88e74b7876d5e025"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.6.1"
   xterm:
     dependency: "direct main"
     description: