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/src/index.ts

@@ -0,0 +1,86 @@
+/**
+ * Storage hub (`ctx.storage`): a named backend registry plus mounted
+ * data-form facilities. The hub itself performs no IO — backends own media,
+ * data forms (the domain layer first) own semantics.
+ * @module @deepseek-ai/dsh-storage
+ */
+
+import { Context, Service } from 'cordis'
+import { StorageError } from './error.ts'
+import { BackendRegistry } from './registry.ts'
+
+export { BackendRegistry } from './registry.ts'
+export { StorageError } from './error.ts'
+export type { StorageErrorCode } from './error.ts'
+export { UNIT_NAME_RE } from './backend.ts'
+export type { StorageBackend, KvFacet, KvUnit, KvUnitDescriptor } from './backend.ts'
+
+declare module 'cordis' {
+  interface Context {
+    storage: Storage
+  }
+}
+
+/**
+ * Data forms mountable on the hub, keyed by form name. Form owners extend
+ * this map via declaration merging (the domain layer merges
+ * `domain: DomainFacility`) and mount the facility in their `apply`.
+ */
+export interface StorageForms {}
+
+/**
+ * The storage hub service. Backends register under `backend`; data forms
+ * mount under their `StorageForms` key and are reached as `ctx.storage.<form>`.
+ */
+export class Storage extends Service {
+  /** Named backend table; multiple backends stay mounted side by side. */
+  readonly backend = new BackendRegistry()
+
+  private readonly forms = new Map<keyof StorageForms, unknown>()
+
+  constructor(ctx: Context) {
+    super(ctx, 'storage')
+  }
+
+  /**
+   * Mount a data-form facility on the hub. Mounting is an effect: the
+   * returned disposer unmounts the form.
+   * @param form - Form key declared in {@link StorageForms}.
+   * @param facility - The facility instance to expose.
+   * @returns the disposer that unmounts the form.
+   */
+  mount<K extends keyof StorageForms>(form: K, facility: StorageForms[K]): () => void {
+    if (this.forms.has(form)) {
+      throw new StorageError('duplicate-mount', `storage form '${String(form)}' is already mounted`)
+    }
+    this.forms.set(form, facility)
+    return () => {
+      this.forms.delete(form)
+    }
+  }
+
+  /**
+   * Resolve a mounted data form.
+   * @param form - Form key declared in {@link StorageForms}.
+   * @returns the mounted facility.
+   */
+  form<K extends keyof StorageForms>(form: K): StorageForms[K] {
+    if (!this.forms.has(form)) {
+      throw new StorageError('form-not-mounted', `storage form '${String(form)}' is not mounted`)
+    }
+    return this.forms.get(form) as StorageForms[K]
+  }
+
+  /** Domain data form; present once the domain layer plugin is loaded. */
+  get domain(): StorageForms extends { domain: infer D } ? D : never {
+    return this.form('domain' as keyof StorageForms) as StorageForms extends { domain: infer D } ? D : never
+  }
+}
+
+/**
+ * Mount the storage hub service.
+ * @param ctx - Plugin context.
+ */
+export function apply(ctx: Context) {
+  ctx.plugin(Storage)
+}