cleanup: derive argon cap directly in library_api range test

The range-decrypt test filled max_argon_memory_mib by building a whole
DecryptOptions::default() and reading one field out of it, which runs
thread and memory detection just to extract a u32. The library already
exports default_argon_decrypt_cap_mib(), which is the value that
default actually uses and says what is meant - call it directly.

Test-only change.

Test plan: cargo test --test library_api passes both tests.

tests/library_api.rs

@@ -9,6 +9,7 @@ use fcry::{
     SecretBytes32,
     decrypt,
     decrypt_range,
+    default_argon_decrypt_cap_mib,
     encrypt,
 };
 use tempfile::TempDir;
@@ -73,7 +74,7 @@ fn library_range_decrypt_raw_key() {
         output_file: Some(out.clone()),
         offset: 1234,
         length: 20_000,
-        max_argon_memory_mib: DecryptOptions::default().max_argon_memory_mib,
+        max_argon_memory_mib: default_argon_decrypt_cap_mib(),
         output: OutputOptions::default(),
     };
     decrypt_range(&range_options, Some(&key), None).unwrap();