Granular Elixir targets

Elixir projects can now define [[elixir.library]] and [[elixir.binary]] targets. Libraries compile to per-target .beam directories, while binaries emit a launcher script that locates the workspace root at runtime. Compilation can be routed through a long-lived BEAM daemon for performance, falling back to direct elixirc invocation when the daemon is not running. State paths such as the CAS, daemon socket, and materialized script now follow XDG Base Directory conventions.