fix(#42):--smells crash on Erlang source files

[rriley-quorum]

Summary

mix reach.check --smells crashed on any project with a .erl file (#42). This PR closes #42.

Root cause: the Erlang frontend never set :module on function metadata, unlike the Elixir frontend. Effects.infer_local_effects groups functions by module and crashed on Module.split(nil) as soon as it hit an Erlang-sourced function.

The issue suggested two fixes: (a) populate :module in the Erlang frontend, or (b) make the crashing call site tolerate a missing :module. We went with (a) because the codebase already relies on every function_def node carrying a real module identity in many places (call graph, dead code, effects inference, etc.) — lib/reach/frontend/beam.ex already does this same thing for compiled BEAM files. Patching just the one call site would leave that same gap open for the ~150 other places that assume a real module, so it would likely just crash again somewhere else.

Changes:

• lib/reach/frontend/erlang.ex: read the -module(name). attribute and stamp it onto each function_def node.
• lib/reach/effects.ex: short_module_alias/1 also crashed on plain Erlang atoms (no Elixir. prefix); it now leaves those alone.

Test plan

• Added tests confirming Erlang function_def nodes carry :module
• Added a regression test reproducing the effects.ex crash
• Reproduced the original crash with a minimal Mix project containing a .erl file and confirmed mix reach.check --smells now runs cleanly
• Full suite passes: mix test (1235 tests, 0 failures)