diff --git a/docs/design/INDEX.md b/docs/design/INDEX.md
index 446a5ca..c26ecde 100644
--- a/docs/design/INDEX.md
+++ b/docs/design/INDEX.md
@@ -623,9 +623,14 @@ for human exploration. Visualization is only a downstream consumer node on the
 streams.
 **Forbids.** Any UI/pixel knowledge inside the engine.
 **Why.** The LLM drives programmatically, the human visually; a headless core
-serves both. The visual face is the **playground** (C22) — egui-native,
-in-process zero-copy from the SoA columns; it is staged after the runnable
-substrate but is core to aura's identity, not optional.
+serves both. The visual face is the **playground** (C22) — a **web frontend
+served from disk-persisted traces** (revised 2026-06, issue #101: the engine
+writes recorded traces to disk, a browser charts them; supersedes the earlier
+egui-native / in-process-zero-copy-from-SoA direction). It is staged after the
+runnable substrate but is core to aura's identity, not optional. The pivot keeps
+this contract's core intact — and arguably tightens it: a browser reading a
+serialized trace file is a stricter "no UI knowledge in the engine" than egui
+reaching zero-copy into the live SoA columns.
 
 ### C15 — Resampling-as-node; sessions/calendars
 **Guarantee.** A resampler is a node (finer stream → coarser bar stream),
@@ -1066,9 +1071,14 @@ load-bearing in the flat graph.
 
 ## Open architectural threads not yet resolved
 
-- **Playground & World UI surface** — the playground is core (C22), egui-native;
-  open is the concrete UI of the meta-views (walk-forward, sweep surfaces,
-  comparison) and the run/replay clock controls.
+- **Playground & World UI surface** — the playground is core (C22); the surface
+  is a **web frontend served from disk-persisted traces** (revised 2026-06, issue
+  #101; not egui). Settled for the first cut: raw per-tap trace persistence to
+  disk (columnar/SoA form, C7) + serve-time `join_on_ts` alignment + a static
+  self-contained HTML chart page (uPlot, vendored like the `render_html`
+  Graphviz-WASM blob), feeds overlaid or timestamp-aligned in panels. Still open:
+  families-comparison meta-views (walk-forward, sweep surfaces, multi-strategy /
+  instrument comparison), a local server, and the run/replay clock controls.
 - **Parameter-space search strategies** (Bayesian/genetic) — pluggable policies
   atop the atomic sim unit (C12), not yet designed.
 - **`aura new` scaffolder, the experiment-builder API, and `Aura.toml`'s