CF6204 test module: explicitly mark the Windows fixture sparse

Michael, Bryan,

Attached is a test-only sparse-file supplement. Apply the included
local CF6204 v6 test-module prerequisite first, then this patch, on
master 8e82d6610e7. The prerequisite keeps the segment-size guard and
excludes the already merged production I/O fixes.

On native ARM64 and emulated x64, the actual module allocates
4,294,971,392 bytes before the change and 131,072 afterward. Logical
size stays 4,294,967,305 bytes and marker readback passes. The
allocation instrumentation is excluded from the patch; figures reflect
the tested NTFS volume.

The regression and three setup groups pass before/after in both
integration builds. Extracted controls verify unsupported-operation
fallback and unexpected-error cleanup. Production I/O and
larger-segment backup coverage remain outside this supplement.

Let me know your thoughts.

Best,

Harrison