Improve randomisation of answers

DESIGN.md

@@ -480,9 +480,10 @@ amateurfunk-anki [--data DIR] [--out DIR] [--seed STR] [--epoch INT]
 - `--seed STR` — deterministic seed for answer shuffling. Omit it
   for the normal study-deck behavior: a fresh seed is generated for
   each build, so answers are reshuffled every time decks are rebuilt.
-- `--epoch INT` — override the package timestamp epoch. By default we
-  derive it from the manifest's `fetched_at`; this flag is mainly for
-  tests and explicit rebuilds.
+- `--epoch INT` — override the package and note-modification timestamp
+  epoch. By default we use the current rebuild time so Anki recognizes
+  reshuffled notes as newer on re-import; this flag is mainly for tests
+  and explicit reproducible builds.
 
 Exit codes: `0` success, `1` configuration / catalog / build error.
 There is no Stage-2 equivalent of the fetcher's `EXIT_BAD_STATE` —
@@ -602,13 +603,14 @@ same `.apkg` bytes out. Determinism rests on three things:
 2. **Stable shuffle.** `randomized_answers()` builds a per-question
    `random.Random` seeded from SHA-256 of
    `f"{cli_seed}:{question_number}"`.
-3. **Stable timestamps.** Every `now` value in the collection (the
-   `mod` columns, the JSON config blob timestamps) is fixed to
-   `build_epoch` — derived from the manifest's `fetched_at`, or
-   overridden via `--epoch`. ZIP member timestamps are also fixed
-   via `ZipInfo(name, zip_datetime(build_epoch))`. Without this
-   last step, the inner SQLite would be identical but the
-   archive's per-entry mtimes would still vary between runs.
+3. **Controlled timestamps.** Every `now` value in the collection (the
+   `mod` columns and JSON config blob timestamps) uses `build_epoch`.
+   A normal rebuild sets it to the current time because the fresh
+   shuffle changes note HTML and Anki only updates an existing note
+   when the imported copy is newer. `--epoch` fixes it explicitly for
+   reproducible builds. ZIP member timestamps use the same value via
+   `ZipInfo(name, zip_datetime(build_epoch))`; otherwise the inner
+   SQLite could be identical while the archive bytes still differed.
 
 The combined effect: two runs with the same `data/`, `--seed`, and
 timestamp inputs produce byte-identical sha256 on each `.apkg`.