Add plugin-driven DSL guards

[dannote]

Summary

• add an optional plugin callback for AST ranges whose syntax is reinterpreted by macros
• suppress generic smell findings and corpus evidence inside those ranges while preserving plugin-owned findings
• cover Ecto query DSLs, Ash expressions/resource blocks, and Nx defn/defnp
• add smells[:dsl_macros] as an escape hatch for unknown DSLs
• register Nx plugin inference and document the configuration

Validation

• MIX_ENV=test mix compile --warnings-as-errors
• mix ci (1000 tests passed)
• mix test test/reach/smell/dsl_guard_test.exs test/reach/evidence/dsl_guard_test.exs test/reach/config_test.exs test/reach/plugin/inference_test.exs test/reach/check/architecture/framework_policy_boundary_test.exs test/reach/check/architecture/plugin_inference_boundary_test.exs
• pinned smell corpus snapshots remained unchanged after explicit regeneration