refactor(fileio): move separate-output routing policy to Rust

Move the separate-destination mode decision into the Rust file-iteration
boundary. The projection now carries the mirror-mode bit and one callback for
each destination policy, so Rust selects the callback once before preserving
the existing non-short-circuiting iteration and aggregate error behavior.

Keep path construction, mirror-directory diagnostics, resource ownership, and
the per-file compression leaf in the opaque C callbacks. Add C/Rust layout
assertions and focused tests proving mirror/flat routing and ordered error
aggregation.

Test Plan:
- `ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --all-targets` (793 tests)
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets` (184 tests)
- `ulimit -v 41943040; make -j1`
- `ulimit -v 41943040; make -j1 -C tests test`

programs/fileio.c

@@ -1166,11 +1166,15 @@ int FIO_rust_compressMultipleFilenames(
 
 typedef int (*FIO_rust_compress_multiple_separate_file_fn)(
         void* opaque, const char* srcFileName);
+/* Rust selects the destination-mode callback; both callbacks retain their
+ * private resources, path construction, diagnostics, and compression leaf. */
 typedef struct {
     void* fCtx;
     const char** inFileNamesTable;
     void* opaque;
-    FIO_rust_compress_multiple_separate_file_fn compressFile;
+    int mirrorOutput;
+    FIO_rust_compress_multiple_separate_file_fn compressMirroredFile;
+    FIO_rust_compress_multiple_separate_file_fn compressFlatFile;
 } FIO_rust_compress_multiple_separate_projection_t;
 typedef char FIO_rust_compress_multiple_separate_fctx_offset[
         (offsetof(FIO_rust_compress_multiple_separate_projection_t, fCtx) == 0) ? 1 : -1];
@@ -1180,12 +1184,22 @@ typedef char FIO_rust_compress_multiple_separate_input_names_offset[
 typedef char FIO_rust_compress_multiple_separate_opaque_offset[
         (offsetof(FIO_rust_compress_multiple_separate_projection_t, opaque)
          == 2 * sizeof(void*)) ? 1 : -1];
-typedef char FIO_rust_compress_multiple_separate_callback_offset[
-        (offsetof(FIO_rust_compress_multiple_separate_projection_t, compressFile)
+typedef char FIO_rust_compress_multiple_separate_mode_offset[
+        (offsetof(FIO_rust_compress_multiple_separate_projection_t, mirrorOutput)
          == 3 * sizeof(void*)) ? 1 : -1];
+typedef char FIO_rust_compress_multiple_separate_mirrored_callback_offset[
+        (offsetof(FIO_rust_compress_multiple_separate_projection_t, compressMirroredFile)
+         == 4 * sizeof(void*)) ? 1 : -1];
+typedef char FIO_rust_compress_multiple_separate_flat_callback_offset[
+        (offsetof(FIO_rust_compress_multiple_separate_projection_t, compressFlatFile)
+         == 5 * sizeof(void*)) ? 1 : -1];
+typedef char FIO_rust_compress_multiple_separate_int_size[
+        (sizeof(int) <= sizeof(void*)) ? 1 : -1];
+typedef char FIO_rust_compress_multiple_separate_callback_size[
+        (sizeof(FIO_rust_compress_multiple_separate_file_fn) == sizeof(void*)) ? 1 : -1];
 typedef char FIO_rust_compress_multiple_separate_projection_size[
         (sizeof(FIO_rust_compress_multiple_separate_projection_t)
-         == 3 * sizeof(void*) + sizeof(FIO_rust_compress_multiple_separate_file_fn)) ? 1 : -1];
+         == 6 * sizeof(void*)) ? 1 : -1];
 int FIO_rust_compressMultipleSeparateFilenames(
         const FIO_rust_compress_multiple_separate_projection_t* projection);
 
@@ -3517,30 +3531,34 @@ typedef struct {
     int compressionLevel;
 } FIO_rust_compress_multiple_separate_context_t;
 
-static int FIO_rust_compressMultipleSeparateFileCallback(void* opaque,
-                                                          const char* srcFileName)
+static int FIO_rust_compressMultipleSeparateMirroredFileCallback(void* opaque,
+                                                                  const char* srcFileName)
 {
     FIO_rust_compress_multiple_separate_context_t* const context =
         (FIO_rust_compress_multiple_separate_context_t*)opaque;
-    const char* dstFileName;
-
-    if (context->outMirroredRootDirName) {
-        char* const validMirroredDirName = UTIL_createMirroredDestDirName(
-            srcFileName, context->outMirroredRootDirName);
-        if (validMirroredDirName) {
-            dstFileName = FIO_determineCompressedName(
-                srcFileName, validMirroredDirName, context->suffix);
-            free(validMirroredDirName);
-        } else {
-            DISPLAYLEVEL(2, "zstd: --output-dir-mirror cannot compress '%s' into '%s' \n",
-                         srcFileName, context->outMirroredRootDirName);
-            return 1;
-        }
-    } else {
-        dstFileName = FIO_determineCompressedName(
-            srcFileName, context->outDirName, context->suffix);
+    char* const validMirroredDirName = UTIL_createMirroredDestDirName(
+        srcFileName, context->outMirroredRootDirName);
+    if (validMirroredDirName) {
+        const char* const dstFileName = FIO_determineCompressedName(
+            srcFileName, validMirroredDirName, context->suffix);
+        free(validMirroredDirName);
+        return FIO_compressFilename_srcFile(
+            context->fCtx, context->prefs, *context->ress,
+            dstFileName, srcFileName, context->compressionLevel);
     }
 
+    DISPLAYLEVEL(2, "zstd: --output-dir-mirror cannot compress '%s' into '%s' \n",
+                 srcFileName, context->outMirroredRootDirName);
+    return 1;
+}
+
+static int FIO_rust_compressMultipleSeparateFlatFileCallback(void* opaque,
+                                                              const char* srcFileName)
+{
+    FIO_rust_compress_multiple_separate_context_t* const context =
+        (FIO_rust_compress_multiple_separate_context_t*)opaque;
+    const char* const dstFileName = FIO_determineCompressedName(
+        srcFileName, context->outDirName, context->suffix);
     return FIO_compressFilename_srcFile(
         context->fCtx, context->prefs, *context->ress,
         dstFileName, srcFileName, context->compressionLevel);
@@ -3598,7 +3616,9 @@ int FIO_compressMultipleFilenames(FIO_ctx_t* const fCtx,
         };
         FIO_rust_compress_multiple_separate_projection_t projection = {
             fCtx, inFileNamesTable, &callbackContext,
-            FIO_rust_compressMultipleSeparateFileCallback
+            outMirroredRootDirName != NULL,
+            FIO_rust_compressMultipleSeparateMirroredFileCallback,
+            FIO_rust_compressMultipleSeparateFlatFileCallback
         };
         if (outMirroredRootDirName)
             UTIL_mirrorSourceFilesDirectories(inFileNamesTable, (unsigned)fCtx->nbFilesTotal, outMirroredRootDirName);