feat(cli): move gzip result classification to Rust

The gzip codec loop already returns distinct status values, but the C
wrapper still owned the status branch that selected the CLI failure path.
That kept a format-result policy in the implementation-bearing C file even
though the loop itself is Rust. Add a Rust diagnostic mapping for success,
initialization, deflate, finish, end, invalid projection, and unknown
statuses. C now branches on the Rust classification while retaining zlib
result values, EXM_THROW codes, and exact diagnostic strings. This keeps the
ABI and observable behavior unchanged and leaves unrelated source-exclusion
and zstd classification seams untouched.

Test Plan:
- Nightly rustfmt check on fileio_asyncio.rs -- passed.
- Targeted git diff checks -- passed.
- Capped GCC syntax-only compile of fileio.c with codec defines -- passed.
- Focused Rust mapping tests were added; Cargo/tests were not run per the
  task's OOM constraint.

rust/src/fileio_asyncio.rs

@@ -994,6 +994,29 @@ pub const FIO_RUST_GZIP_FINISH_ERROR: c_int = 3;
 pub const FIO_RUST_GZIP_END_ERROR: c_int = 4;
 pub const FIO_RUST_GZIP_INVALID_PROJECTION: c_int = 5;
 
+pub const FIO_RUST_GZIP_DIAGNOSTIC_OK: c_int = 0;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_INIT_ERROR: c_int = 1;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_DEFLATE_ERROR: c_int = 2;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_FINISH_ERROR: c_int = 3;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_END_ERROR: c_int = 4;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_INVALID_PROJECTION: c_int = 5;
+pub const FIO_RUST_GZIP_DIAGNOSTIC_UNKNOWN: c_int = 6;
+
+/// Classifies gzip compression results while leaving zlib error values and
+/// the CLI exception text in the C callback.
+#[no_mangle]
+pub extern "C" fn FIO_rust_gzipCompressionDiagnostic(status: c_int) -> c_int {
+    match status {
+        FIO_RUST_GZIP_OK => FIO_RUST_GZIP_DIAGNOSTIC_OK,
+        FIO_RUST_GZIP_INIT_ERROR => FIO_RUST_GZIP_DIAGNOSTIC_INIT_ERROR,
+        FIO_RUST_GZIP_DEFLATE_ERROR => FIO_RUST_GZIP_DIAGNOSTIC_DEFLATE_ERROR,
+        FIO_RUST_GZIP_FINISH_ERROR => FIO_RUST_GZIP_DIAGNOSTIC_FINISH_ERROR,
+        FIO_RUST_GZIP_END_ERROR => FIO_RUST_GZIP_DIAGNOSTIC_END_ERROR,
+        FIO_RUST_GZIP_INVALID_PROJECTION => FIO_RUST_GZIP_DIAGNOSTIC_INVALID_PROJECTION,
+        _ => FIO_RUST_GZIP_DIAGNOSTIC_UNKNOWN,
+    }
+}
+
 const FIO_RUST_GZIP_Z_OK: c_int = 0;
 const FIO_RUST_GZIP_Z_STREAM_END: c_int = 1;
 const FIO_RUST_GZIP_Z_BUF_ERROR: c_int = -5;
@@ -5271,6 +5294,39 @@ mod tests {
         }
     }
 
+    #[test]
+    fn gzip_compression_status_diagnostic_preserves_cli_mapping() {
+        for (status, diagnostic) in [
+            (FIO_RUST_GZIP_OK, FIO_RUST_GZIP_DIAGNOSTIC_OK),
+            (
+                FIO_RUST_GZIP_INIT_ERROR,
+                FIO_RUST_GZIP_DIAGNOSTIC_INIT_ERROR,
+            ),
+            (
+                FIO_RUST_GZIP_DEFLATE_ERROR,
+                FIO_RUST_GZIP_DIAGNOSTIC_DEFLATE_ERROR,
+            ),
+            (
+                FIO_RUST_GZIP_FINISH_ERROR,
+                FIO_RUST_GZIP_DIAGNOSTIC_FINISH_ERROR,
+            ),
+            (FIO_RUST_GZIP_END_ERROR, FIO_RUST_GZIP_DIAGNOSTIC_END_ERROR),
+            (
+                FIO_RUST_GZIP_INVALID_PROJECTION,
+                FIO_RUST_GZIP_DIAGNOSTIC_INVALID_PROJECTION,
+            ),
+        ] {
+            assert_eq!(FIO_rust_gzipCompressionDiagnostic(status), diagnostic);
+        }
+
+        for status in [-1, 99] {
+            assert_eq!(
+                FIO_rust_gzipCompressionDiagnostic(status),
+                FIO_RUST_GZIP_DIAGNOSTIC_UNKNOWN
+            );
+        }
+    }
+
     #[test]
     fn lzma_compression_status_diagnostic_preserves_cli_mapping() {
         for (status, diagnostic) in [