feat(storage): storage hub with named backend registry and data-form mounts

ctx.storage is a pure registration hub: multiple named backends stay
mounted side by side, data forms (domain first) mount via the
merge-extensible StorageForms map. src/backend.ts is the normative
KV-facet contract; tests/contract.ts is the shared conformance suite
every backend runs. Backends expose data-shape facets (kv now, an
append-log facet reserved for the future session-backend migration).

packages/storage/storage/tsconfig.json

@@ -0,0 +1,21 @@
+{
+  "extends": "../../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "lib/types"
+  },
+  "include": [
+    "src"
+  ],
+  "references": [
+    {
+      "path": "../../../vendor/cosmokit"
+    },
+    {
+      "path": "../../../vendor/cordis"
+    },
+    {
+      "path": "../../support/invariants"
+    }
+  ]
+}