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_server/config/generator.yaml

@@ -0,0 +1,16 @@
+type: server
+
+client_package_path: ../halkyon_learn_client
+server_test_tools_path: test/integration/test_tools
+
+# Private and shared data sync through serverpod_offline_sync (ADR 0001): its module,
+# and the experimental `database: sync` for the models it syncs.
+modules:
+  serverpod_offline_sync:
+    nickname: offline_sync
+  # What every Halkyon server has (ADR 0014): `client.modules.base` in the app.
+  halkyon_base:
+    nickname: base
+
+experimental_features:
+  databaseSync: true