Task: Bring tenor_convention_resolution's messaging layer under codegen

(Describe what user-visible-or-internal change this task produces.)

• tenor_convention_resolution regenerates cleanly (or the model documents why it stays hand-authored) without breaking registrar.cpp.
• If brought under codegen, the hand-wired inline block in registrar.cpp is replaced by the generated sub-registrar, not left duplicated.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.