Fix card-data download for Scryfall's new gzipped-JSONL bulk API

"Download card data" had been failing on every platform with "missing field
`download_uri`". Scryfall changed the bulk-data API: entries now carry
`jsonl_download_uri`/`compressed_size` instead of `download_uri`/`size`, and
the payload is gzipped JSONL (one card object per line) rather than an
uncompressed JSON array. An existing reference.db keeps working, which is why
this went unnoticed.

- Rename the BulkEntry fields and decompress with flate2 (the response is
  Content-Type: application/gzip with no Content-Encoding, so ureq won't
  transparently inflate it). rust_backend keeps it pure Rust, so nothing new
  has to cross-compile for Android.
- Replace the streaming JSON-array visitor with a line-based JSONL parser.
  Closure errors now propagate directly instead of being laundered through
  serde::de::Error::custom, so cancellation reports a clean Cancelled;
  override_if_cancelled stays as a safety net and its doc comment is corrected.
- Progress wraps the compressed stream, since compressed_size is what the index
  reports.
- Test fixtures stay readable pretty-printed JSON arrays via a jsonl() helper
  that converts them, so tests still exercise the production parser.

Downloads are far smaller now: 101MB compressed (24 + 77) versus ~740MB, which
matters a lot on a phone.

Also log command failures to the platform log. Tauri command errors are
returned to the WebView as values, so they never reached logcat and the UI only
showed them truncated -- this bug was invisible until then. mlog! is a plain
eprintln!, which tao's Android glue forwards to logcat under RustStdoutStderr,
so no logging dependency is needed. View with: adb logcat -s RustStdoutStderr

Verified against the live API: 36,242 oracle cards, 116,409 printings, trigram
FTS working, and the aarch64 APK still builds.

Cargo.lock

@@ -1832,6 +1832,7 @@ name = "mtgc-core"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "flate2",
  "nucleo",
  "rusqlite",
  "serde",