feat: target-ontology-adapter §3 TargetOntologyAdapter protocol

[deanban]

TargetOntologyAdapter PEP 544 Protocol with version-aware registry, entry-point discovery, and import-boundary enforcement (adapters cannot reach into sema.engine/sema.pipeline/sema.graph, transitively).

Tracking: #97. Source: openspec/changes/target-ontology-adapter/tasks.md §3.

Tasks

• 3.1 TargetOntologyAdapter @runtime_checkable Protocol
• 3.2 Default iter_terms mixin raising NotImplementedError
• 3.3 register_target_adapter decorator (PEP 440 specifier strings)
• 3.3a Add packaging to dependencies
• 3.4 registry.get with version-aware resolution + error taxonomy
• 3.5 registry.list
• 3.6 Overlap-rejection guard at registration
• 3.7 discover_entry_points (no auto-registration)
• 3.8 Import-boundary AST test
• 3.9 Allow-list for permitted imports
• 3.9a Transitive-closure boundary test
• 3.10 isinstance runtime-check test
• 3.11 Missing-method registration rejection test
• 3.12 Registry error-taxonomy tests
• 3.12a Non-overlapping OMOP version-range routing test
• 3.13 Boundary test against fixture importing sema.engine
• 3.13a Transitive boundary test