platform: posix: Correct posix_fuzz_sz to size_t to prevent truncation

The fuzzer's payload size posix_fuzz_sz is provided by libFuzzer as
a size_t. Declaring it as a uint8_t in the ipc test harness resulted
in silent payload truncation (maximum 255 bytes) causing incomplete
corpus generation. This corrects the types between fuzz.c and ipc.c.

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo

src/platform/posix/fuzz.c

@@ -11,6 +11,8 @@
 #include <nsi_cpu_if.h>
 #include <nsi_main_semipublic.h>
 
+#include <platform/posix_fuzz.h>
+
 const uint8_t *posix_fuzz_buf;
 size_t posix_fuzz_sz;
 

src/platform/posix/include/platform/posix_fuzz.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright(c) 2026 Intel Corporation. All rights reserved.
+ */
+
+/**
+ * Symbols shared between the native_posix fuzz harness
+ * (src/platform/posix/fuzz.c) and the SOF posix IPC layer
+ * (src/platform/posix/ipc.c).
+ *
+ * Defining them in one place avoids type-mismatch bugs (e.g. a single
+ * `extern uint8_t *posix_fuzz_buf, posix_fuzz_sz;` declaring `posix_fuzz_sz`
+ * as a `uint8_t` rather than a `size_t`).
+ */
+
+#ifndef PLATFORM_POSIX_FUZZ_H
+#define PLATFORM_POSIX_FUZZ_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+extern const uint8_t *posix_fuzz_buf;
+extern size_t posix_fuzz_sz;
+
+#endif /* PLATFORM_POSIX_FUZZ_H */

src/platform/posix/ipc.c

@@ -24,7 +24,7 @@ static void posix_ipc_isr(void *arg)
 }
 
 // External symbols set up by the fuzzing layer
-extern uint8_t *posix_fuzz_buf, posix_fuzz_sz;
+#include <platform/posix_fuzz.h>
 
 // Lots of space.  Should really synchronize with the -max_len
 // parameter to libFuzzer (defaults to 4096), but that requires