refactor(cli): move zstd status classification into Rust

The zstd compression stream already runs its scheduling and accounting loop in
Rust, but the C callback still owned the result-status switch that selected
success, codec failure, incomplete input, or invalid projection handling. Move
that status-to-diagnostic policy into the Rust ABI module, matching the
existing LZMA and LZ4 diagnostic seams. C keeps the zstd-specific error-name
lookup, display text, and exception construction, so private codec details and
CLI diagnostics remain on the C side and the observable error behavior is
unchanged. Unknown statuses retain the projection-error fallback.

Test Plan:
- `rustfmt +nightly --edition 2021 --check rust/src/fileio_asyncio.rs` -- passed.
- `git diff --check` and `git diff --cached --check` -- passed.
- Capped GCC syntax-only check of `programs/fileio.c` with all CLI format
  feature defines -- passed.
- Cargo, make, native builds, and large tests were not run per worker OOM rules.

rust/src/fileio_asyncio.rs

@@ -822,6 +822,25 @@ pub const FIO_RUST_ZSTD_COMPRESS_ERROR: c_int = 1;
 pub const FIO_RUST_ZSTD_INCOMPLETE_INPUT: c_int = 2;
 pub const FIO_RUST_ZSTD_INVALID_PROJECTION: c_int = 3;
 
+pub const FIO_RUST_ZSTD_DIAGNOSTIC_OK: c_int = 0;
+pub const FIO_RUST_ZSTD_DIAGNOSTIC_COMPRESS_ERROR: c_int = 1;
+pub const FIO_RUST_ZSTD_DIAGNOSTIC_INCOMPLETE_INPUT: c_int = 2;
+pub const FIO_RUST_ZSTD_DIAGNOSTIC_INVALID_PROJECTION: c_int = 3;
+pub const FIO_RUST_ZSTD_DIAGNOSTIC_UNKNOWN: c_int = 4;
+
+/// Classifies zstd compression results while leaving zstd-specific error
+/// names and CLI exception text in the C callback.
+#[no_mangle]
+pub extern "C" fn FIO_rust_zstdCompressionDiagnostic(status: c_int) -> c_int {
+    match status {
+        FIO_RUST_ZSTD_OK => FIO_RUST_ZSTD_DIAGNOSTIC_OK,
+        FIO_RUST_ZSTD_COMPRESS_ERROR => FIO_RUST_ZSTD_DIAGNOSTIC_COMPRESS_ERROR,
+        FIO_RUST_ZSTD_INCOMPLETE_INPUT => FIO_RUST_ZSTD_DIAGNOSTIC_INCOMPLETE_INPUT,
+        FIO_RUST_ZSTD_INVALID_PROJECTION => FIO_RUST_ZSTD_DIAGNOSTIC_INVALID_PROJECTION,
+        _ => FIO_RUST_ZSTD_DIAGNOSTIC_UNKNOWN,
+    }
+}
+
 const FIO_RUST_ZSTD_E_CONTINUE: c_int = 0;
 const FIO_RUST_ZSTD_E_END: c_int = 2;
 const UTIL_FILESIZE_UNKNOWN: u64 = u64::MAX;
@@ -5321,6 +5340,34 @@ mod tests {
         }
     }
 
+    #[test]
+    fn zstd_compression_status_diagnostic_preserves_cli_mapping() {
+        for (status, diagnostic) in [
+            (FIO_RUST_ZSTD_OK, FIO_RUST_ZSTD_DIAGNOSTIC_OK),
+            (
+                FIO_RUST_ZSTD_COMPRESS_ERROR,
+                FIO_RUST_ZSTD_DIAGNOSTIC_COMPRESS_ERROR,
+            ),
+            (
+                FIO_RUST_ZSTD_INCOMPLETE_INPUT,
+                FIO_RUST_ZSTD_DIAGNOSTIC_INCOMPLETE_INPUT,
+            ),
+            (
+                FIO_RUST_ZSTD_INVALID_PROJECTION,
+                FIO_RUST_ZSTD_DIAGNOSTIC_INVALID_PROJECTION,
+            ),
+        ] {
+            assert_eq!(FIO_rust_zstdCompressionDiagnostic(status), diagnostic);
+        }
+
+        for status in [-1, 99] {
+            assert_eq!(
+                FIO_rust_zstdCompressionDiagnostic(status),
+                FIO_RUST_ZSTD_DIAGNOSTIC_UNKNOWN
+            );
+        }
+    }
+
     const SOURCE_POLICY_STAT: u8 = 1;
     const SOURCE_POLICY_EXCLUDED: u8 = 2;
     const SOURCE_POLICY_OPEN: u8 = 3;