Add explanations

DESIGN.md

@@ -632,6 +632,22 @@ byte-identical sha256 on each `.apkg`. Verified during review.
   the card background. We inject a white `<rect>` as the first
   painted element when packaging the SVG into the `.apkg`. The
   on-disk extracted files are left untouched.
+- **Explanations layer (optional).** `amateurfunk_anki.py` loads
+  `explanations.json` (CLI: `--explanations`) and, for any
+  question number found there, appends an English explanation
+  block to the back of the card. The block is styled distinctly
+  (serif italic body, sans-serif metadata, top border) and shows
+  a small "low confidence" badge when the entry's `confidence`
+  field is below `LOW_CONFIDENCE_THRESHOLD` (= 7). `revision` and
+  the raw `confidence` number are editorial-only and never
+  displayed. A missing file is treated as an empty database; a
+  malformed entry is a hard `AnkiBuildError`. The editorial
+  contract (schema, sourcing, agent workflows) lives in
+  `EXPLANATIONS.md`; this file documents only the wiring on the
+  build side. Note-GUID stability across explanation edits: the
+  GUID is keyed on `category.slug:number`, not on field content,
+  so adding/editing an entry updates the existing Anki note on
+  re-import rather than producing a duplicate.
 
 ### Testing focus
 
@@ -667,6 +683,13 @@ the embedded SQLite, and check structural invariants.
 - Deterministic build: two consecutive builds against the same
   catalog with a fixed `--epoch` produce byte-identical `.apkg`
   files.
+- Explanations layer: a present entry produces a styled
+  explanation block on the back card; a missing entry leaves the
+  card unchanged; `confidence < 7` surfaces the "low confidence"
+  badge; URL sources render as `<a>`, citation sources render as
+  escaped text; a missing `explanations.json` is silently empty;
+  a malformed entry (bad type, out-of-range confidence, missing
+  field) raises `AnkiBuildError`.
 
 ### What we deliberately do NOT do in Stage 2