This PR adds `GenerateTrie(db, scheme, root)` to the `triedb` package,
which rebuilds all tries from flat snapshot KV data. This is needed by
snap/2 sync so it can rebuild the trie after downloading the flat state.
The shared trie generation pipeline from `pathdb/verifier.go` was moved
into `triedb/internal/conversion.go` so both `GenerateTrie` and
`VerifyState` reuse the same code.
