Substrate-crossing seam — instrumented governed crossing

Run 4 (baseline). Published under a Keyhive-gated crossing-intent record per PC#8 v0.1.3 (write-before-fire). This record carries a seamCrossingRef back-pointer (Item 1.4 / KL-2) referencing the governed Automerge source document. This record is a KL-1/KL-2 evidence target — do not delete until the closing-evidence artifact is produced.