feat: enforce supply chain policy

docs/SUPPLY-CHAIN.md

@@ -0,0 +1,24 @@
+# Multiplayer artifact supply chain
+
+Container references in the repository are immutable `@sha256:` digests. The
+base manifests may contain a zero digest only as a deployment template; a
+release overlay must replace it with a registry-resolved digest and run the
+checker with `--require-concrete`.
+
+The release pipeline must, for every image and exported server artifact:
+
+1. generate and retain an SBOM tied to the exact digest;
+2. scan OS and application dependencies and fail on a critical or disallowed
+   vulnerability;
+3. sign the image and provenance with the offline release authority, and
+   verify both at cluster admission; and
+4. publish the digest, SBOM, scan result, signature and provenance as one
+   immutable release record.
+
+Critical vulnerability fixes are triaged immediately and a patched release is
+cut within 24 hours of confirmation. A release with an unaccepted critical
+finding or unverifiable signature is not eligible for admission.
+
+`python3 scripts/verify_supply_chain.py` is the dependency-free repository
+guard. Registry signing/scanning and admission require the release environment
+and are intentionally not simulated by this local check.