[antono3]

Summary

Pass mapped pointer values into a small byte-store helper rather than casting heap-promoted voidptr locals in place. This avoids the compiler bug reported with a dependency-free single-file example in vlang/v#31481. Add a CPU-only regression verifying byte writes and pointer identity for escaped pointer slots.

Verification

• Current V3 bfc309e17e7e8fd23f4c1e2444b8d8646ec13ddf and TinyCC: both buffer-example test files pass.
• The original native buffer example reproducibly fails with pointer difference 533 instead of 512. The patch completes on GTX 1060 with validation enabled: shared mappings, dedicated image, live budgets, upload-ring wrap and fenced GPU copy/readback (21,22,23).
• The helper changes only this example; allocator library API, dependency pins, version and generated bindings are unchanged.
• Cross-platform and sanitizer CI must pass before merge.