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/migrations/migration_registry.txt

@@ -0,0 +1,35 @@
+### AUTOMATICALLY GENERATED DO NOT MODIFY
+###
+### This file is generated by Serverpod when creating a migration, do not modify it
+### manually. If a collision is detected in this file when doing a code merge, resolve
+### the conflict by removing and recreating the conflicting migration.
+
+20260916165410927
+20260930094729362-studysets
+20260930122235111-cards
+20260930145136285-reviews
+20261001064926911-tree-tags-assets
+20261002035419149-file-allowance
+20261002191122122-notes
+20261002211346701-learning-paths
+20261003110855698-pdf-sources
+20261007205440868-card-buried-at
+20261008165051407-pdf-position
+20261008223835206-dictionaries
+20261009123850448-dictionary-kind
+20261009145434785-dictionary-entry-version
+20261009184924904-public-profiles
+20261009195137836-published-studysets
+20261009202105673-explore
+20261009215552429-learner-count
+20261009220324372-based-on
+20261009224606274-feedback
+20261010095547719-assistant-connector
+20261012162411835-studyset-deleted-at
+20261013035552510-dictionary-keys
+20261014153932647-release-colour
+20261015171326465-email-sign-in
+20261015220121195-verified-links
+20261015220626303-discussions
+20261017110130653
+20261019121406578-halkyon-base