[dmsdio 2/9] Migrate dmfsi, dmvfs, and dmdevfs to 64-bit I/O #7

Parent: #4
Depends on: #6

Goal

Carry 64-bit offsets and sizes through the complete VFS and device-filesystem path.

Work

• Move dmfsi seek, stat, and directory-entry sizes to explicit 64-bit types and API 2.0.
• Migrate dmvfs file positions, seek/tell/stat, mount dispatch, and SAL bindings.
• Migrate dmdevfs handle offsets and dmdrvi dispatch to API 2.0.
• Add checked conversion at any unavoidable legacy libc boundary.
• Update dmffs, dmramfs, testfs, and filesystem tests as required.

Acceptance criteria

• A synthetic device larger than 4 GiB can be sought, read, written, and stat-ed through /dev.
• Boundary tests cover INT32_MAX, UINT32_MAX, and positions above 4 GiB.
• Overflow returns an explicit error instead of wrapping.