feat(engine): disclose at plan time whether an apply continues or discards a copy

[aparajon]

Why this matters

The engine can already predict whether an apply will resume unfinished work on the target or destroy it, but only at apply time — the moment the decision is unavoidable. By then the copy is gone. The point of predicting it at all is to tell someone before they confirm, and that requires the plan to carry the answer.

What it does

Runs the same prediction at plan time, against the target the plan already reads, and carries the result on the plan as an engine-agnostic ExistingCopy per namespace.

plan
  └─ for each namespace the plan touches
       └─ does a copy exist for a table in this batch?
            ├─ no  → PlanResult carries nothing
            └─ yes → PlanResult.ExistingCopies += { namespace, disposition, reason, tables, age }

The shape is one entry per namespace, not one per plan, so an engine that spans several targets reports which one each disclosure is about. A single-database engine produces at most one and a caller planning several concatenates them.

Two safety properties worth calling out:

• A plan is a read. It describes the target and must never fail because of it. A target that cannot be read is logged and the plan carries no disclosure, leaving the plan exactly as it is without the check.
• A plan-time miss is not the last word. The same prediction runs again at apply time against the routing the apply actually took, so a target that was unreachable while planning is still evaluated before anything is destroyed.

The type carries no engine-specific fields — namespace, disposition, reason, tables, age — so a second engine that copies tables reports the same shape and every surface renders it one way.

How it moves us toward the northstar

This is the middle link of three. The engineer whose schema change is about to destroy a day of copying learns nothing from an engine's internal state — they read a plan, in a PR comment or on the CLI. The plan is the one object every surface already reads, so putting the disposition there is what lets any of them disclose it without reaching into an engine. #1048 makes the prediction; #1088 renders it.

Opened by Claude (Opus 5).