feat(paths): path files carry tables, callouts and furigana, and several import at once

The import format gains the blocks lessons need: tables (rows of cells),
callouts (a kind, a title, blocks inside, but no table or callout),
spoilers, quotes and dividers; and runs of text gain a reading shown as
furigana, a colour, underline and strikethrough. A line break is a "\n"
in the text. Still simple blocks turned into rich text here, so a change
of the editor's own format never breaks a file written before. A table's
columns are as wide as their longest line, narrowed alike to fit a
phone, where the cells wrap.

A studyset's Import › Learning paths now takes several files at once,
one path each, stored in the order of their names, so a chapter's
lessons come in with one pick. Every file is read before anything is
stored: one that can't be read is named, and nothing comes in.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/support/fake_file_picker.dart

@@ -6,6 +6,9 @@ class FakeFilePicker() implements FilePicker {
   /// What the next pick gives; null as if the person cancelled.
   PickedFile? next;
 
+  /// What the next pick of several gives; [next] alone if null.
+  List<PickedFile>? nextMany;
+
   /// The extensions of each pick asked for, in order.
   final asked = <List<String>>[];
 
@@ -17,4 +20,13 @@ class FakeFilePicker() implements FilePicker {
     asked.add(extensions);
     return next;
   }
+
+  @override
+  Future<List<PickedFile>> pickMany({
+    required String label,
+    required List<String> extensions,
+  }) async {
+    asked.add(extensions);
+    return nextMany ?? [?next];
+  }
 }