chore(changeset): spec minor for the declared dataset-answer object

Clause-② is yes (widening): AnalyticsResult and its response schema gain
one optional member.

Claude-Session: https://claude.ai/code/session_01Sfe5YjBLwB9J3y8fvm2xq1
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/20647-analytics-result-object.md

@@ -0,0 +1,23 @@
+---
+'@objectstack/spec': minor
+---
+
+feat(spec): a dataset answer declares its base object as `object` (#20647)
+
+Clause-②: yes (widening)
+
+`AnalyticsResult` (`@objectstack/spec/contracts`) gains one optional member,
+`object?: string`, and `AnalyticsResultResponseSchema` (`@objectstack/spec/api`)
+mirrors it on `data`. It is the base object of the dataset the answer was
+computed from, by machine name. Nothing is removed or renamed, and no existing
+member changes meaning.
+
+**For a consumer.** Code typed against `AnalyticsResult` can read `object` from a
+`queryDataset` answer without a cast, and a parse with
+`AnalyticsResultResponseSchema` keeps `data.object` where it used to strip it. The
+contract asks every dataset answer to carry it, whatever dimensions are selected
+and whether or not rows came back. A cube query answer has no dataset behind it
+and carries none.
+
+**Producers.** This release declares the member. `@objectstack/service-analytics`
+sets it on every dataset answer once #20644 lands.