Align E NotLoaded diagnostics with structured model locations and JSON Pointer #244

Follow the existing checker ObjectLocation contract and confirmed clarification in teaql/teaql-code-gen#300: retain canonical KSML segments internally, expose RFC 6901 instancePath under the selected wire naming profile, and render Java-native diagnostics separately. Carry structured paths through E traversal, including list indices and unavailable relation details; preserve NotLoaded and no implicit I/O. Add narrow positive/negative tests and update the owned School example. Local verification only; no publication or tag push. Java work is stacked on draft #243, without changing its reviewed head.