What is an Oracle ASSEMBLY?
There is absolutely nothing about this on the Internet - even the Oracle docs regarding the %_ASSEMBLIES views don't actually state what they show.

However, if you go digging around in $ORACLE_HOME/rdbms/admin you'll see the following reference to the data dictionary views (which sit on assembly$:

remark
remark  FAMILY "ASSEMBLIES"
remark
remark  Views for showing information about PL/SQL Assemblies:
remark  USER_ASSEMBLIES, ALL_ASSEMBLIES and DBA_ASSEMBLIES
remark
create or replace view USER_ASSEMBLIES
(ASSEMBLY_NAME, FILE_SPEC, SECURITY_LEVEL, IDENTITY, STATUS)
.......


This definitely says to me that these views, and the associated CREATE/DROP/ALTER ASSEMBLY commands, are for .NET stored procedure support.