docs(engine): point the depth-sign comment at the ticket that would cover it

The comment already admits the self-verify net cannot catch a re-inversion
here. Three reviewers asked for the render-parity comp that could. That is not
a fixture drop — the harness has no channel for `HF_FAST_CAPTURE_3D`, the
golden has to be produced on the Linux/Docker CI platform, and the sibling
fast-capture fixture is itself excluded from every shard — so it is scoped in
PRINFRA-570 and named here, along with the fact that until it lands this sign
has no CI coverage at all.

packages/engine/src/services/threeDProjection.ts

@@ -816,7 +816,11 @@ export async function initThreeDProjectionInPage(): Promise<ThreeDProjectionResu
       // rendering a two-plane preserve-3d comp on both capture paths and
       // comparing; the self-verify net CANNOT catch a regression here, because
       // it captures ground truth after this rewrite and would compare wrong
-      // geometry against wrong geometry.
+      // geometry against wrong geometry (measured: a 65 dB "pass" over a
+      // truly-broken 18.8 dB render). The only backstop that can see this class
+      // of regression is a render-parity comp in a regression shard, which
+      // needs harness plumbing rather than a fixture drop — PRINFRA-570 carries
+      // the scope. Until it lands, THIS SIGN IS UNCOVERED BY CI.
       const ndc: Mat4 = [
         2 / canvasW,
         0,