feat(privacy): every synced payload sealed and opened through one envelope (ADR 0019)

- `ContentKeys` opens the person's data keys (`content_key`) with their
  account's keys on the device, an anonymous person's device keys made as
  their data first opens; it makes a private studyset's key and the
  personal key. Without the account's keys nothing opens and nothing new
  is private (D8).
- `PayloadEnvelope` is the one place payloads are sealed and opened, each
  for its table, row and column, so a payload moved to another row never
  opens; one that doesn't open is `Locked`, never half-read.
- `Privacy` gives each gateway the key of the row's tree (its top-level
  studyset's), and rewrites a tree under another key, or readable, in
  place (`rekeyTree`).
- The studyset, card, note, learning path, path answer and PDF gateways
  seal with their tree's key and open what they read: a new top-level
  studyset is private where the device holds the keys, a new one inside a
  tree takes its privacy, and one moved into another tree is rewritten
  under that tree's key. Tags and settings are sealed with the personal
  key; a tag or settings row that doesn't open here is left out, so
  nothing folds or deletes it. A studyset whose key doesn't open shows as
  locked (`Studyset.isLocked`); other rows that don't open read as blank.
- Every payload codec round-trips sealed, and refuses another row.

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_content_key_gateway.dart

@@ -0,0 +1,29 @@
+import 'dart:typed_data';
+
+import 'package:halkyon_learn_flutter/src/core/privacy/data/content_key_gateway.dart';
+
+/// [ContentKeyGateway] in memory; several devices of one person share one,
+/// as their sync would.
+class FakeContentKeyGateway() implements ContentKeyGateway {
+  /// The keys kept.
+  final List<ContentKeyRow> rows = [];
+
+  @override
+  Future<List<ContentKeyRow>> all() async => [...rows];
+
+  @override
+  Future<void> insert({
+    required String dataKeyId,
+    required ContentKeyKind kind,
+    required String? studysetId,
+    required Uint8List sealedKey,
+    required DateTime createdAt,
+  }) async => rows.add((
+    id: 'key-${rows.length + 1}',
+    dataKeyId: dataKeyId,
+    kind: kind,
+    studysetId: studysetId,
+    sealedKey: sealedKey,
+    createdAt: createdAt,
+  ));
+}

apps/learn/halkyon_learn_flutter/test/support/fake_studyset_gateway.dart

@@ -26,6 +26,10 @@ class FakeStudysetGateway() implements StudysetGateway {
   /// How often the studysets were read.
   int reads = 0;
 
+  /// Whether a new top-level studyset is private, as on a device holding
+  /// the account's keys (ADR 0019).
+  bool privateByDefault = false;
+
   /// Stores [payload] as if another device had written it and a sync had
   /// brought it here, with the other columns given.
   void receiveRemoteChange(
@@ -37,6 +41,8 @@ class FakeStudysetGateway() implements StudysetGateway {
     DateTime? deletedAt,
     DateTime? starredAt,
     DateTime? updatedAt,
+    bool isPrivate = false,
+    bool locked = false,
   }) {
     _rows[id] = (
       id: id,
@@ -47,6 +53,8 @@ class FakeStudysetGateway() implements StudysetGateway {
       deletedAt: deletedAt,
       starredAt: starredAt,
       updatedAt: updatedAt,
+      isPrivate: isPrivate,
+      locked: locked,
     );
     _remoteChanges.add(null);
   }
@@ -58,6 +66,43 @@ class FakeStudysetGateway() implements StudysetGateway {
     _remoteChanges.add(null);
   }
 
+  /// Every change of privacy, in order, as (id, private).
+  final privacyChanges = <(String, bool)>[];
+
+  @override
+  bool canMakePrivate = true;
+
+  @override
+  Future<int> setPrivate(String id, {required bool private}) async {
+    final tree = {id};
+    var grew = true;
+    while (grew) {
+      grew = false;
+      for (final row in _rows.values) {
+        if (tree.contains(row.parentId) && tree.add(row.id)) {
+          grew = true;
+        }
+      }
+    }
+    for (final member in tree) {
+      final row = _rows[member]!;
+      _rows[member] = (
+        id: row.id,
+        payload: row.payload,
+        parentId: row.parentId,
+        position: row.position,
+        archivedAt: row.archivedAt,
+        deletedAt: row.deletedAt,
+        starredAt: row.starredAt,
+        updatedAt: row.updatedAt,
+        isPrivate: private,
+        locked: row.locked,
+      );
+    }
+    privacyChanges.add((id, private));
+    return 0;
+  }
+
   @override
   Future<List<StudysetRow>> all() async {
     reads++;
@@ -84,6 +129,13 @@ class FakeStudysetGateway() implements StudysetGateway {
       deletedAt: null,
       starredAt: null,
       updatedAt: updatedAt,
+      // New studysets are private where the device can encrypt (ADR 0019):
+      // the parent's tree's privacy, or [privateByDefault] at the top.
+      isPrivate: switch (parentId) {
+        final parent? => _rows[parent]?.isPrivate ?? false,
+        null => privateByDefault,
+      },
+      locked: false,
     );
     writes.add((id, payload));
     return id;
@@ -105,6 +157,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: row.deletedAt,
         starredAt: row.starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
       writes.add((id, payload));
     }
@@ -126,6 +180,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: row.deletedAt,
         starredAt: starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
     }
   }
@@ -147,6 +203,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: row.deletedAt,
         starredAt: row.starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
       moves.add((id, parentId, position));
     }
@@ -168,6 +226,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: row.deletedAt,
         starredAt: row.starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
       moves.add((id, row.parentId, position));
     }
@@ -189,6 +249,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: row.deletedAt,
         starredAt: row.starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
     }
   }
@@ -209,6 +271,8 @@ class FakeStudysetGateway() implements StudysetGateway {
         deletedAt: deletedAt,
         starredAt: row.starredAt,
         updatedAt: updatedAt,
+        isPrivate: row.isPrivate,
+        locked: row.locked,
       );
     }
   }