Overmorrow — host capability probe

Deployment-target contract verification for ticket #54, discharging D8 of the build-toolchain decision (#12). This is a fixture, not the product.

What the Node harness checks separately

Header round-trip, byte-identical serving against the integrity manifest, and per-path cache policy are asserted from outside the browser by probe/scripts/assert-contract.mjs. This page covers only what a browser can prove: that the policies are enforced, not merely present.