InspectorGraph: Pick<Graph, "read" | "explain" | "subscribe" | "now">

Capability handed to inspect and watchInspect. Closes #257 (capability narrowing across factory/selector seams): the inspector reads explanation lineage, subscribes to its updates, and stamps each view with graph.now. Those four methods — read, explain, subscribe, now — are the entire surface.

commit, input, derived, hydrate, snapshot, exportModel, subscribeCommits, commitLog, readAt/snapshotAt are intentionally unreachable: an inspector that can mutate the engine ceases to be inspection and becomes a parallel writer, breaking the "engine is inspectable through its own primitives, not a parallel devtools system" commitment.

Narrowing is type-level. A real Graph is still assignable, so call sites keep working; the discipline is enforced at compile time inside the inspector implementation.