feat(launcher): stabilize prepared content updates

docs/architecture/worldbuilder-inventory.md

@@ -132,6 +132,20 @@ bake/equivalence/UI-Studio tools, not a production fallback. Portal → HighRes
 installed-DAT gates are recorded in
 `docs/research/2026-07-24-slice-c-prepared-asset-cutover-report.md`.
 
+**Launcher cumulative-overlay extension (2026-08-25).** Production still has
+no live-DAT fallback and consumes the same prepared-payload contracts. For a
+bounded recipe migration, App and Headless may receive one complete base pak
+plus one cumulative filtered pak through `LayeredPreparedAssetSource`. The
+overlay is probed first: Missing falls through to the base, while a present but
+corrupt render or collision payload remains authoritative corruption. Both
+mapped owners share one composite lifetime and there is never an overlay
+chain. The launcher binds the overlay to the base digest in the optional
+`pak/content.current.json` sidecar; format/global extraction migrations retain
+the explicit full-rebuild path. A tiny `pak/content.client-pending` marker
+keeps either result non-launchable until the matching client is confirmed,
+including across a crash/restart. Design and gates:
+`docs/plans/2026-08-25-launcher-content-stabilization.md`.
+
 **Slice I3 prepared collision extension (2026-07-25).** The package remains
 format 1 and retains mesh type values 1–3; bake-tool 4 appends typed GfxObj,
 Setup, CellStruct, and EnvCell-topology collision payloads. Core owns the