feat: generate build and supply-chain evidence

docs/security/supply-chain.md

@@ -0,0 +1,18 @@
+# Build and supply-chain gate
+
+Merge and release controls:
+
+- frozen `pnpm-lock.yaml` installation; drift is blocking
+- clean production build with hashed assets and build manifest
+- machine-readable bundle sizes and checksums
+- source plus built-asset credential-pattern scan
+- direct dependency inventory and lockfile digest
+- base/head dependency diff review record
+
+Organization-specific vulnerability severity, denied-license list, SBOM format,
+and scanner selection remain policy inputs. An approved suppression must record
+reason, owner, expiry, affected package, and compensating control. Expired
+suppressions are blocking.
+
+`artifacts/security/dependency-diff.json` is a local baseline. CI replaces it
+with the actual base/head direct and transitive lockfile diff before release.