Concept
Layers interoperate by contract, not by coupling
The formats compose one-directionally and dependency-light: they agree by contract name, most are zero-dependency and vendored, so a layer can be replaced without touching the ones above it.
Pinned to
web4/1 — the nine-layer stack mapThe canonical source this concept is taught from. If it moves, the lesson is regenerated — teaching is pinned to code, not retyped.
On the frontier
No course teaches this concept yet — it is a named gap on the content graph, not a hole nobody sees. A gap is the roadmap, not an embarrassment: the count only falls by teaching.