docs(web-ui/shape): add routes header, trim historical framing

docs/web-ui/shape.md

@@ -3,6 +3,8 @@
 > Part of [Web UI](../web-ui.md). See also:
 > [Dashboard layout](dashboard.md) · [Per-agent page](agent.md)
 
+## Shared routes
+
 - `GET /` → `index.html` from the bundled frontend dist (see
   `frontend/`). Both binaries' routers declare their dynamic
   endpoints first and then `fallback_service(ServeDir::new(...))`
@@ -19,8 +21,8 @@
   `dashboard.css` / `flow.css` / `logs.css`); `common.css` inlines
   `base.css` + `terminal.css` via esbuild's `@import` resolution.
   `terminal.js` exports `{ create, linkify }` as ES module
-  members (no more `window.HiveTerminal` global outside the
-  back-compat shim the IIFE bodies still use). The dashboard's
+  members; a back-compat shim in the IIFE bodies still exposes
+  a `window.HiveTerminal` global for callers that need it. The dashboard's
   `#msgflow` and the per-agent `#live` log are both backed by
   this terminal — sticky-bottom auto-scroll, "↓ N new" pill,
   history backfill, SSE plumbing all live there. Each page
@@ -209,13 +211,11 @@ soon as they blur.
 **Atomic section repaint:** every managed-section renderer goes
 through `paintAtomic(liveRoot, build)`: the builder appends into
 a fresh `DocumentFragment` (off-DOM) and the commit is one
-`replaceChildren` call. The naive `root.innerHTML = ''; root.append(...)`
-shape was visibly flashing empty on every poll cycle — on async
-paths the await yield gave the browser a paint opportunity between
-the clear and the re-append, and on complex builds (many `el()`
-allocations) layout could escape the per-task budget even on the
-synchronous path. The fragment approach keeps the intermediate
-empty state invisible. Builders receive the fragment as their
+`replaceChildren` call, so the intermediate empty state is never
+visible — clearing and re-appending directly into the live root can
+leave it empty for a paint (on async builders) or a whole frame (on
+large synchronous ones), which reads as a visible flash on every
+poll cycle. Builders receive the fragment as their
 `root`, so existing renderer code carries over unchanged; early
 returns inside the builder still commit whatever was appended
 before they returned.