Cloze deletion

[Aduneer]

Wrap a word in {{braces}} and the card is asked with that word taken out.

The {{mitochondrion}} is the powerhouse of the cell,,biology

┌──────────────────────────────────────────┐
│ Box 1  ·  new  ·  biology                │
├──────────────────────────────────────────┤
│                                          │
│   The [...] is the powerhouse of the cell│
│                                          │
└──────────────────────────────────────────┘

It is a way of writing a card, not a new kind of card: one Leitner box, one
id, one due date, and tags, hints, undo, editing, images and the log all work
on it unchanged. New module src/cloze.*; still zero dependencies.

The deck format did not change, but it did gain a floor

The answer column is empty, because the answers live inside the question. That
makes a cloze card a whole card on a single field, so:

• card_from_csv accepts a one-field line when — and only when — it holds a
  real deletion. Without that test every line of every text file would be a
  card, and the rule that keeps a mistyped path from being overwritten depends
  on that not happening.
• card_to_csv lowers its three-column floor to one for such a card.

Both halves are needed, or a deck of bare sentences would be rewritten as
sentence,, the first time it was opened — the same rule that already keeps a
question,answer,tags deck from growing trailing commas. No new column, so
nothing about this can put a file-synced deck in conflict between versions.

The one syntax decision

Anki's syntax, so decks paste across in both directions: {{text}},
{{c1::text}}, {{c1::text::hint}}, and | alternatives inside a deletion.

The hint separator collides with the :: a C++ deck is made of. Resolved by
looking for a hint only after a cN:: prefix:

{{std::vector}}          → answer "std::vector"
{{c1::std::vector}}      → answer "std", hint "vector"   (Anki's own reading)
{{c1::std::vector::}}    → answer "std::vector", no hint

Several holes: one card, one schedule

They are asked one at a time within a single presentation, each finished blank
filled in for the next, with what it earned kept on screen underneath.

│ Box 1  ·  new  ·  geography  ·  blank 2 of 3             │
│   Paris is the capital of [...], on the [river]          │

  ✅  blank 1  Paris

The card is then scheduled once, on the worst of the answers. The
alternative — one prompt per hole, each moving the box — would promote a
three-blank card three boxes in one sitting. One review, one log event,
whatever the sentence is made of.

Structurally this is why the review loop now walks a vector<Ask> per card
instead of a single expected answer. That also collapsed the reversed and
normal paths into one thing built up front rather than rediscovered inside the
prompt loop, and it is what makes the hint/quit/audio keys withheld per
blank rather than per card.

Two deliberate exclusions

• Never asked reversed. A cloze card's answers are already inside its
  question, so there is nothing to turn round. A reversed session asks such
  cards forwards — which is what lets a mixed deck be studied backwards
  without splitting it in two — and logs them as n, because that is how they
  were actually asked.
• --generate-audio skips them. A recording is of the question, and a
  cloze question read aloud is the answer read aloud; there is also no single
  rendering to make, since a sentence with three holes is asked three ways
  against a column that holds one file. The a key still works during review:
  the open blank is spoken as the word "blank", and the whole sentence is read
  once the card is done.

Tests

make check is now 1,344 unit checks and 58 golden cases (was 1,233 / 50).
Clean under gcc and clang with -Werror and -fsanitize=address,undefined -fno-sanitize-recover=undefined, under the env -i clean room, and from a
fresh clone.

Six new golden cases: a single blank; three blanks with a hint and a grouped
repeat; a reversed session over a mixed deck; undo restoring a part-answered
card; per-blank key withholding on a vim deck whose first blank is q; and
--generate-audio skipping one.

A seventh is unrelated and was in the way: the in-app help screen (h) had
no end-to-end coverage at all — the existing help case is --help, which
is a different screen printed by different code. menu-help was added before
print_help was touched.

Also: examples/cloze-science.csv (12 cards, round-trips byte for byte), the
manage list shows a cloze card as it will be asked rather than as a row of
braces, and adding or editing one stops asking for an answer it would never
use.