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.

src-tauri/src/lib.rs

@@ -7,6 +7,30 @@ use tauri::Manager;
 
 use state::AppState;
 
+/// Write a line to the platform log.
+///
+/// On Android this reaches `adb logcat` under the tag `RustStdoutStderr`: tao's
+/// Android glue `dup2`s stdout/stderr into a pipe and forwards each line to the
+/// system log, so a plain `eprintln!` is all that's needed and no logging
+/// dependency is warranted. On desktop it's just stderr.
+///
+/// This matters because Tauri command failures are *returned* to the WebView as
+/// values, not raised — so without logging them explicitly they never appear in
+/// logcat at all, and the UI only ever shows a truncated version.
+#[macro_export]
+macro_rules! mlog {
+    ($($arg:tt)*) => { eprintln!("[mtgc] {}", format_args!($($arg)*)) };
+}
+
+/// Log a command failure (with its full, untruncated message) and pass the
+/// result through unchanged.
+pub(crate) fn logged<T>(what: &str, result: Result<T, String>) -> Result<T, String> {
+    if let Err(e) = &result {
+        crate::mlog!("{what} failed: {e}");
+    }
+    result
+}
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()
@@ -17,6 +41,7 @@ pub fn run() {
                 .app_data_dir()
                 .expect("resolve app data dir");
             std::fs::create_dir_all(&data_dir).ok();
+            mlog!("data dir: {}", data_dir.display());
             app.manage(Mutex::new(AppState::new(data_dir)));
             Ok(())
         })