chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_curator/pubspec.yaml

@@ -0,0 +1,19 @@
+name: halkyon_learn_curator
+description: >-
+  Turns dictionary downloads (JMdict, and the JLPT lists on top) into the
+  dictionary files a Halkyon Learn server loads.
+publish_to: none
+
+environment:
+  sdk: '^3.13.0'
+
+resolution: workspace
+
+dependencies:
+  archive: ^4.0.7
+  args: ^2.7.0
+  crypto: ^3.0.7
+  xml: ^7.0.1
+
+dev_dependencies:
+  test: ^1.25.5