Top-level integer decode traps on empty token

[dduan]

Fuzzing found a crash with this input:

-=0

Minimal repro:

try TOMLDecoder().decode(Int.self, from: "-=0\n")

Expected: decoding throws.
Actual: integer unpacking can trap because the top-level keyed document leaves the decoder with Token.empty.

[dduan]

This no longer reproduces on current main (30d03ac). I reran the exact minimal case:

try TOMLDecoder().decode(Int.self, from: "-=0\n")

It now throws DecodingError instead of trapping. This was fixed by #331, which rejects top-level single-value decoding when the parsed TOML document is a table.