fix(privacy): what the app shows and adds keeps private studysets private (ADR 0019)

The reading of 22 Oct, on the device's side. Private cards never move
into a readable studyset: the card browser asks to make it private first.
Locked studysets are left out of the move sheets and the destinations
words are added to. A studyset whose key isn't on this device says to
sign in with the password here, not that it is lost, and can still be
archived; creating one there says why it can't. The marker shows the lock
for a private studyset published before, and a shield with a warning
while something in it is still readable or unsealed, with Finish in its
dialog. The library's offer covers every top-level studyset, Recently
deleted and orphaned ones too, says what couldn't be sealed, and its
answer follows the person in their settings (schema 11).

The full-window editors carry the studyset they were opened from, and an
account with keys adds no file where none is known (a new studyset's
description takes pictures once it is made). A file no live row names is
never uploaded, and deleting a row clears what it owed. Sealed rows are
indexed by key and hash. A suggestion from a private copy uploads
readable copies of its files for the author; accepting swaps whole
`asset:` values only.

Docs: ADR 0019's "The reading (22 Oct 2026)" with the decisions, what
changed and the residual risks; "As built" and Learn's AGENTS.md brought
up to date.

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_flutter/test/support/fake_studyset_gateway.dart

@@ -74,6 +74,9 @@ class FakeStudysetGateway() implements StudysetGateway {
   @override
   bool canMakePrivate = false;
 
+  @override
+  bool keysMissing = false;
+
   @override
   Future<int> setPrivate(String id, {required bool private}) async {
     final tree = {id};