docs(adr): how step 3's last features are offered, D53–D56

Explaining a miss in a snack bar after Bad, checking as a wrong typed
reading's Why? and Check in the field, cards from text as drafts in a
sheet, made from a note, a PDF page or pasted text.

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

docs/adr/0021-private-instance-and-assistant.md

@@ -5,7 +5,7 @@
   keeps and does, where it lives, the proposal's points 4–8 and 10, how often it warns and
   when it is built (D13–D18), who signs up and who operates (D19–D22), waiting accounts
   (D23–D26), what people are told and what never goes (D27–D30), how the first step is
-  built (D31–D34), the second (D35–D46) and the third (D47–).
+  built (D31–D34), the second (D35–D46) and the third (D47–D56).
 - **Builds on:** ADR 0016 (the register), ADR 0019 (private studysets), ADR 0020
   (SimpleLogin), ADR 0022 (the hand-off and the connector gone).
 - **Changes:** the rule of 10 Oct 2026 that Halkyon calls no language model with an API
@@ -334,6 +334,24 @@ On 26 Oct, how the third step is built (the assistant answering in Learn):
   no assistant there): where the account's keys aren't open, the sheet says the
   conversation isn't kept, and it is gone once the sheet closes; nothing unsealed is
   stored or synced, as with own keys.
+- **D53 Explaining a miss is offered in a snack bar after Bad** (recommended, over the card
+  waiting after Bad with Explain and Next, and over Explain beside the answer before
+  rating): rated Bad, a snack bar offers Explain for a few seconds, which opens the
+  conversation sheet over the next card, explaining the card just rated from its own
+  parts; a session that doesn't use it goes on as fast as before.
+- **D54 Checking is a wrong typed reading's Why? and Check in the field** (recommended,
+  over a sentence exercise on vocabulary cards and over either part alone): a typed
+  reading checked wrong offers Why? beside it; the field offers Check beside Ask, which
+  checks what the person typed as writing in the studyset's language (with the card as
+  context in a session) and says what is wrong and why, changing nothing.
+- **D55 Cards made from text are drafts in a sheet** (recommended, over one draft at a
+  time in the composer): listed as the text import lists cards, each chosen or not, a
+  tap opening one in the composer to change it; adding takes those chosen, marked as
+  made by an assistant (`origin: assistant`); nothing is added before (D10).
+- **D56 Cards are made from a note, a PDF page or pasted text** (the three recommended,
+  over asking in the field as well): a note's menu (the whole note, or the text
+  selected), the PDF reader's card panel (the page shown, its text as PDFium reads it),
+  and Add material › With an assistant (a paragraph pasted or typed).
 
 ## Proposal