Task: Junction codegen template follow-ups

Four codegen template issues for junction entities: path/namespace mismatch between generated and live code, current_timestamp version-collision in insert triggers, messaging handler generated without matching service, and eventing facets produce incomplete output.

(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.