Import Anki decks with their files and history

Files in content: the person's images and sounds as asset rows synced like
all content, their files kept by hash on each device, uploaded after each
sync, downloaded on demand and checked against their hash
(core/asset/AssetRepository). Rich text names them as asset:<id> URLs, in
the editor's image block and an audio block of ours; any other URL is never
fetched. Sounds play through audioplayers, one at a time. Content holding
only an image or a sound isn't blank, and lists show it as such.

The Anki import ("Import from Anki" under Add material): .apkg and .colpkg
packages in all three formats Anki has written, read on an isolate of their
own. Modern packages compress everything with zstd in frames that don't
record their size, which pure_zstd v1.0.0 refuses; our fork decodes them.
Fields become rich text by a converter of our own (lines, lists, formatting,
images, sounds, cloze deletions hidden on the front). One card per note,
the files first, studysets as chosen (flatten, sub-studysets, or tags by
subdeck), the notes' tags, and, if wanted, the review history, which FSRS
replays: the first template's reviews to the card's schedule, a reversed
note type's second to the reverse one.

Imported cards say where they came from in their first revision (card
schema 3); the dashboard doesn't count them as written. The test fixtures
are exported by Anki itself (tool/anki/make_fixtures.py).

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

pubspec.lock

@@ -90,6 +90,62 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.13.1"
+  audioplayers:
+    dependency: transitive
+    description:
+      name: audioplayers
+      sha256: "2ba4bb2944baacbdd5372ff8254a8e7feb8c10d7739545e392f5605a8f618745"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.8.1"
+  audioplayers_android:
+    dependency: transitive
+    description:
+      name: audioplayers_android
+      sha256: f5ff5b15620fbab8cb0849e9636c48e2b96c3f0f71723bbbe2ad3c761b205f05
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.3.0"
+  audioplayers_darwin:
+    dependency: transitive
+    description:
+      name: audioplayers_darwin
+      sha256: "1ca553add991384ecf421b9569da850f3ab2472ffb83f6970b0416365abc51be"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.5.0"
+  audioplayers_linux:
+    dependency: transitive
+    description:
+      name: audioplayers_linux
+      sha256: "15178b726b7cdee5364d0463c8d445630c4e0fb7d26612b73c767e7d25de9417"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.3.0"
+  audioplayers_platform_interface:
+    dependency: transitive
+    description:
+      name: audioplayers_platform_interface
+      sha256: "765f6f0e6dca55cb471c9483fc77700564b3484d19198aca4ebb5147c6c85acb"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.2.0"
+  audioplayers_web:
+    dependency: transitive
+    description:
+      name: audioplayers_web
+      sha256: ae1e0103c865a03e273f6d13d97b93f5595eac09915729cd5e37ef96e2857319
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.3.0"
+  audioplayers_windows:
+    dependency: transitive
+    description:
+      name: audioplayers_windows
+      sha256: a70ae82bba2dfcb6eb03dd4815d737a2d46d33ea5a96a03f535cfcaac490e413
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.4.1"
   barcode:
     dependency: transitive
     description:
@@ -418,6 +474,70 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.0.0"
+  file_selector:
+    dependency: transitive
+    description:
+      name: file_selector
+      sha256: bd15e43e9268db636b53eeaca9f56324d1622af30e5c34d6e267649758c84d9a
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.0"
+  file_selector_android:
+    dependency: transitive
+    description:
+      name: file_selector_android
+      sha256: d670cd0ce77a2e785b18d8b4d0a8d6a222d6a813ec9b7ddf2790a1b4fb6fa92c
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.2+11"
+  file_selector_ios:
+    dependency: transitive
+    description:
+      name: file_selector_ios
+      sha256: "97269e5307a0ab813b1fa2430bada0a96e0afb74848417f8676f64ba5de0051c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.3+6"
+  file_selector_linux:
+    dependency: transitive
+    description:
+      name: file_selector_linux
+      sha256: da76400e7872ce7637ffdce12749ec24169c25f6195c28372208e65a24bcd2ab
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.4+1"
+  file_selector_macos:
+    dependency: transitive
+    description:
+      name: file_selector_macos
+      sha256: d57c62362766b5e7ae739448650b66c6aab7a68ba7ecc65e04018652645ae0f4
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.5+1"
+  file_selector_platform_interface:
+    dependency: transitive
+    description:
+      name: file_selector_platform_interface
+      sha256: "35e0bd61ebcdb91a3505813b055b09b79dfdc7d0aee9c09a7ba59ae4bb13dc85"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.7.0"
+  file_selector_web:
+    dependency: transitive
+    description:
+      name: file_selector_web
+      sha256: "73181fbc5257776d8ecaa6a94ab3c8e920ad143b9132a6d984a9271dfc6928d3"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.5"
+  file_selector_windows:
+    dependency: transitive
+    description:
+      name: file_selector_windows
+      sha256: fbefc5fb92c6d3cbe8d284a2cd971b593bb07d2cd6da8557b81a862250b4acec
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.3+6"
   fixnum:
     dependency: transitive
     description:
@@ -1096,6 +1216,15 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.2.1"
+  pure_zstd:
+    dependency: transitive
+    description:
+      path: "."
+      ref: "596b63d5741d479073fbfc78377cc7dbdd342da3"
+      resolved-ref: "596b63d5741d479073fbfc78377cc7dbdd342da3"
+      url: "https://code.halkyon.app/project-halkyon/pure-zstd.git"
+    source: git
+    version: "1.0.0"
   qr:
     dependency: transitive
     description: