Panic in `StructFields::fields()` on malformed file: unwraps a fallible DType decode

[lorenzhs]

What happened?

More fuzzing results. Opening a crafted .vortex file whose file DType is a struct with a field whose DType-union
discriminant is unknown panics. The public open path (VortexOpenOptions::open) calls into StructFields::fields(), which vortex_expect(...)s a fallible flatbuffer decode. Field-DType validity is checked lazily, so the top-level DType segment parses fine and the panic only fires later, while parsing the file-statistics footer.

Backtrace:
   0: vortex_array::dtype::serde::flatbuffers::<impl core::convert::TryFrom<vortex_array::dtype::serde::flatbuffers::ViewedDType> for vortex_array::dtype::DType>::try_from
   1: vortex_array::dtype::struct_::FieldDTypeInner::value
   2: <core::iter::adapters::GenericShunt<I,R> as core::iter::traits::iterator::Iterator>::next
   3: vortex_file::footer::file_statistics::FileStatistics::from_flatbuffer
   4: vortex_file::footer::deserializer::FooterDeserializer::deserialize
   5: vortex_file::open::VortexOpenOptions::open::{{closure}}
   6: vortex_file::open::VortexOpenOptions::open_path::{{closure}}
   7: vortex_tui::inspect::VortexInspector::read_footer::{{closure}}
   8: vortex_tui::inspect::exec_inspect::{{closure}}
   9: vortex_tui::native_cli::launch_from::{{closure}}
  10: vx::main::{{closure}}
  11: vx::main
  12: std::sys::backtrace::__rust_begin_short_backtrace
  13: std::rt::lang_start::{{closure}}
  14: std::rt::lang_start_internal
  15: main
  16: __libc_start_call_main
             at ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
  17: __libc_start_main_impl
             at ./csu/../csu/libc-start.c:360:3
  18: _start

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <core::result::Result<T,E> as vortex_error::VortexExpect>::vortex_expect::{{closure}}
   3: <core::iter::adapters::GenericShunt<I,R> as core::iter::traits::iterator::Iterator>::next
   4: vortex_file::footer::file_statistics::FileStatistics::from_flatbuffer
   5: vortex_file::footer::deserializer::FooterDeserializer::deserialize
   6: vortex_file::open::VortexOpenOptions::open::{{closure}}
   7: vortex_file::open::VortexOpenOptions::open_path::{{closure}}
   8: vortex_tui::inspect::VortexInspector::read_footer::{{closure}}
   9: vortex_tui::inspect::exec_inspect::{{closure}}
  10: vortex_tui::native_cli::launch_from::{{closure}}
  11: vx::main::{{closure}}
  12: vx::main

Steps to reproduce

https://4z2.de/fuzz_dtype.vortex sha256sum: 0730a2948c2632c5f462049d0bf6d120fa8d42a2856962f31b6e57818cde444f

vx inspect does the trick

Environment

reproes on develop as of c5e075a

Additional context

No response