fix(spill): declare the parked result image defined to Valgrind Memcheck

The result spill writes each parked result as an opaque image: the record
header (a struct copy) and the compacted arena block. Both contain struct
padding that no code writes, and the image is read back whole, never
interpreted byte by byte. MemorySanitizer builds already declare the image
defined before the fwrite. Valgrind Memcheck flags the same bytes at
write(2) ("Syscall param write(buf) points to uninitialised byte(s)"),
with origins in stack-built records such as CBMReadWrite in
handle_readwrites and the import record in py_ns_import. Zeroing those
records one at a time only moves the report to the next record type.

When <valgrind/memcheck.h> is available (non-Windows, __has_include),
SPILL_IMAGE_DEFINED now issues VALGRIND_MAKE_MEM_DEFINED. Outside Valgrind
that is a few no-op instructions. Builds without the header compile it to
nothing as before.

Verified: extraction:extract_spill_round_trip_keeps_every_field under
Memcheck went from 1 error context to 0. The extraction suite passes
(470), and the -Werror build and lint pass.

Author: ngallodev

internal/cbm/result_spill.c

@@ -29,11 +29,20 @@
  * image that is read back whole and never interpreted byte by byte (CI MSan
  * lane on #2202: offset 4087, then 6714, of a 6,952-byte block). Under MSan
  * the image is declared defined right before the write; every other build
- * compiles this to nothing. */
+ * compiles this to nothing. Valgrind Memcheck flags the same padding at the
+ * write(2), so when its client-request header is available the image is
+ * declared defined to it too (a few no-op instructions outside Valgrind). */
 #include "foundation/sanitized.h" /* __has_feature exists everywhere, cppcheck included */
 #if __has_feature(memory_sanitizer)
 #include <sanitizer/msan_interface.h>
 #define SPILL_IMAGE_DEFINED(p, n) __msan_unpoison((p), (n))
+#elif !defined(_WIN32) && defined(__has_include)
+#if __has_include(<valgrind/memcheck.h>)
+#include <valgrind/memcheck.h>
+#define SPILL_IMAGE_DEFINED(p, n) ((void)VALGRIND_MAKE_MEM_DEFINED((p), (n)))
+#else
+#define SPILL_IMAGE_DEFINED(p, n) ((void)0)
+#endif
 #else
 #define SPILL_IMAGE_DEFINED(p, n) ((void)0)
 #endif