Fix heap snapshots containing >256 node types (#61809)

I was getting some very silly results where `CodeInstance` nodes were
`UnitRange{Int64}` and `Core.DebugInfo` was `Compiler.AnalysisResults`,
etc.

Bug found by claude. Probably introduced in #52854, when this field went
from `size_t` to `uint8_t`.

~~todo: tests~~ would need to implement a parser for heapsnapshot
files....