The Construction-layer milestone's anchor spec: a Blueprint graph-as-data that
compiles (inlines composites) to the flat (nodes, sources, edges) the unchanged
Harness::bootstrap consumes (C9/C19/C23). The SMA-cross composite runs
bit-identical to the hand-wired sample_harness (C1). Non-goals: optimisation
passes, external deps, named-handle ergonomics, aura graph.
Grounding-check PASS (13/13 current-behaviour assumptions ratified by green tests).
refs #12
