refactor(storage): rename dsh-domain to dsh-storage-domain

The bare 'domain' name was too generic for a published package. The
directory moves to packages/storage/storage-domain, the package becomes
@deepseek-ai/dsh-storage-domain, and the plugin/invariant names follow;
the ctx surface (ctx.storage.domain), the domain/changed event, and all
runtime behavior are unchanged. References, catalogs, graphs, and the
bilingual design note move together.

packages/storage/storage-domain/src/error.ts

@@ -0,0 +1,53 @@
+/**
+ * Error vocabulary of the domain data form.
+ * @module @deepseek-ai/dsh-storage-domain/src/error
+ */
+
+/** Discriminant codes carried by every {@link DomainError}. */
+export type DomainErrorCode =
+  | 'already-open'
+  | 'facet-unsupported'
+  | 'invalid-record'
+  | 'missing-key'
+  | 'closed'
+
+/** Location of the record that failed schema validation at the durable boundary. */
+export interface InvalidRecordDetail {
+  /** Table holding the rejected record; `''` for the global singleton. */
+  readonly table: string
+  /** Key of the rejected record; `''` for the global singleton. */
+  readonly key: string
+}
+
+/** Construction options: standard `cause` plus the `invalid-record` location. */
+export interface DomainErrorOptions extends ErrorOptions {
+  /** Present exactly when `code` is `invalid-record`. */
+  readonly detail?: InvalidRecordDetail
+}
+
+/**
+ * Error thrown by the domain layer. The `code` is the stable contract
+ * consumers may switch on; `message` is diagnostic prose. Backend failures
+ * (`backend-not-found`, `version-mismatch`, …) pass through as
+ * `StorageError` — the domain layer does not rewrap them.
+ */
+export class DomainError extends Error {
+  override readonly name = 'DomainError'
+
+  /** Present exactly when `code` is `invalid-record`. */
+  readonly detail?: InvalidRecordDetail
+
+  /**
+   * @param code - Stable discriminant for the failure class.
+   * @param message - Human-readable diagnostic detail.
+   * @param options - Standard error options plus the `invalid-record` location.
+   */
+  constructor(
+    readonly code: DomainErrorCode,
+    message: string,
+    options?: DomainErrorOptions,
+  ) {
+    super(message, options)
+    if (options?.detail) this.detail = options.detail
+  }
+}