0547 — Run-side worker structs are named *Instance; the concrete Definition keeps both roles

Context

Every worker kind had a *Definition struct carrying both Declare and Provision, and a *Execution struct with Run. The roadmap raised a four-way Definition/Declarer/Provisioner/Instance split because “Definition and Provisioner are mixed”. Survey showed the mixing is concrete-layer only: the Declarer/Provisioner/Execution interfaces are already split and consumed asymmetrically on purpose (admin holds Declarers, systemmanager holds Provisioners). The durable definition is the worker row itself (name, metadata, target_instances); the struct is the machine that writes and provisions it.

Decision

• Rename-only: every *Execution struct becomes *Instance (CronScheduler/Janitor/CursorAdvancer/MetricsCollector/Manager/ PartitionCount/CompactionReadCost/Base), matching the claimed worker_instance row each holds; files move execution.go -> instance.go; the manager pool follows (instancePool, spawnedInstance).
• worker.Execution survives only as the interface name the Instance structs implement (settled 2026-08-16).
• No concrete Definition/Provisioner split: a data-only Definition would have no consumer — nothing uses a definition without provisioning it, so the split would be a parallel mechanism.

Consequences

• i is now the correct receiver on every run-side struct.
• ConsumerInstance/ProducerInstance read consistently with the worker machinery; no interface named Instance exists, so no seam ambiguates.