Fix MSVC warnings in vendored FatFs and xz-embedded

[dimension-zero]

Summary

Three small fixes that quiet MSVC build warnings in vendored third-party code shipped with the Windows projects. No behavior change — these are explicit casts/widening that document the existing intent.

Fixes

C4319 sign-extension bug — Ventoy2Disk/ff14/source/ff.c:6169
```c

• n = (DWORD)(((b_data + sz_blk - 1) & ~(sz_blk - 1)) - b_data);

• n = (DWORD)(((b_data + sz_blk - 1) & (LBA_t)(sz_blk - 1)) - b_data);
  ```
  `(sz_blk - 1)` produces a 32-bit mask which is then AND'd with a 64-bit `LBA_t`. MSVC zero-extends the high bits, so the mask is wrong on the high half (zeros instead of ones). Casting the operand to `LBA_t` before complementing makes the mask correct.

C4267 narrowing — Plugson/src/Lib/xz-embedded/linux/lib/decompress_unxz.c
3 explicit `(int)` / `(unsigned int)` casts where `size_t` values (`b.in_pos`, `b.out_pos`) are assigned to `int` / `unsigned int` xz protocol fields.

C4267 narrowing — Plugson/src/Lib/xz-embedded/linux/lib/xz/xz_dec_lzma2.c
8 explicit `(uint32_t)` casts at the boundaries between `size_t` arithmetic and `uint32_t` LZMA2 fields.

Notes

• All three files are vendored. The FatFs change is against ff14 source in this tree; if upstream prefers to forward this to the FatFs project, happy to do so.
• The xz-embedded changes mirror the spirit of similar narrowing-cast cleanups in the Linux kernel's tree, but applied locally to keep MSVC happy without touching the kernel's copy.

Stats

• 3 files changed: +12 / −12

Test plan

• `Ventoy2Disk.vcxproj` builds Win32 + x64 Release without C4319 or C4267 warnings
• `VentoyPlugson.vcxproj` builds Win32 + x64 Release without C4267 warnings
• Output binaries are byte-equivalent in behavior (no runtime change — these are no-op casts wrt actual values stored)