docs(release): the 10.4.0 entry, the index-health concept doc, and the API surfaces — written from the tree, not the plan

docs/architecture/index-architecture.md

@@ -723,6 +723,14 @@ async stats(): Promise<Statistics> {
 
 ### 5. Index Rebuilding (Lazy Loading Support)
 
+> **Stale as of 10.4 — "Mode 2: Lazy Loading on First Query" below is
+> RETIRED.** `disableAutoRebuild` no longer defers index construction to a
+> first query; `brain.init()` now runs every needed rebuild to completion
+> before it returns, unconditionally, and a read against a not-serving
+> provider throws a typed `*NotReadyError` instead of rebuilding mid-query.
+> See `docs/concepts/index-health.md` for the current contract. Left below
+> as historical background on the rebuild mechanics.
+
 **Two modes of index loading:**
 
 #### Mode 1: Auto-Rebuild on init() (default)