The arg_getBytes() function returns a pointer to the internal bytes
storage which has layout: [size_t size][actual data bytes]

When writing bytes via f.write(), the code was incorrectly writing
from the start of the buffer (the size_t header) instead of the
actual data.

This fix adds + sizeof(size_t) to skip the header, consistent with
how other code handles bytes data:
- _solana_sbf.c:106 (CPI data)
- _solana_sbf.c:181 (CPI data)
- builtins_sbf.c:1342 (_sbf_builtins_str)
- PikaVM.c:961 (bytes indexing, fixed in 64a521b)

Without this fix, VFS writes would corrupt data by writing the
size header bytes instead of the actual payload.