cuda.core: a fast-path VMM grow leaves its extension unfreeable

Summary

_grow_allocation_fast_path maps a new chunk into a second, adjacent VA reservation and updates buf._size in place. The Buffer's deleter captured the original size when the buffer was created, so close() calls deallocate(ptr, original_size) and frees only the first reservation. The extension's reservation, mapping, and physical memory leak with no warning.

No deallocate(ptr, size) call can fix this: cuMemAddressFree frees a reservation only when ptr and size match exactly one reservation, so a range that spans two reservations cannot be freed in one call. The buffer needs to own each reservation and mapping it consists of.

Status

Today the fast path is unreachable (#2388 defect 2). #2237 and #2407 make it live and would expose this leak, so they should wait for this fix. The existing fast-path test mocks the driver and cannot catch it.

Refs: #2388, #2237, #2407, #2882.

_grow_allocation_fast_path から始めて、Buffer、close()、deallocate(ptr, size) が予約、マッピング、物理メモリをどのように管理しているかを追跡してください。既存の fast-path テストを確認し、close 後も拡張予約が割り当てられたままになっていることを検出するカバレッジを追加してください。完了条件は、Buffer が所有するすべての予約とマッピングが解放されることです。