Native flavor provider contract

Hara L0 is evaluator- and platform-independent. A runtime may install native flavor providers, but portable code does not acquire native semantics merely by using a core form.

A namespace selects a provider explicitly with (:flavor :name). Selection is local to that namespace, is not inherited by required namespaces, and grants no authority. An embedding runtime separately grants reflection/invocation, classpath mutation, and compilation/class-definition capabilities.

Providers receive evaluated values at generic evaluator hooks: namespace clauses, native type and qualified-symbol resolution, construction, member reads/writes/calls, indexed steps, and exception matching. Providers must normalize failures into stable native-flavor errors while preserving the original cause. Unsupported providers and denied capabilities must fail deterministically.

Executable Hara source uses .hal. .hara is retained only for historical descriptors; current projects use executable project.hal descriptors. .hrl and .hara are not executable module extensions.