fix(web): harden the app-origin paths and correct three docs

Copilot review round 1 on #2100.

- POST /api/app-document: `null` and `[1,2]` are valid JSON, so the
  destructure that followed threw OUTSIDE the parse try/catch and turned a
  malformed body into a 500 instead of the documented 400.
- app-origin controller: the error handler cleared `server` but not
  `origin`, and `publish` gates on `origin` — so an error after a successful
  listen kept minting URLs on a dead port and the browser never took the
  srcdoc fallback. Clear both, and drop the now-unfetchable documents.
- sandbox proxy: drop a stale `srcdoc` before navigating to a dedicated
  origin (srcdoc wins over src), and restore the opaque-origin posture —
  strip `allow-same-origin`, reset `innerOrigin` — before assigning `srcdoc`
  on the fallback path. Not reachable today (each render remounts the
  iframe), but the cost of being wrong about that later is an isolation
  boundary.
- publishAppDocument tests: replace two `as unknown as Response` casts with
  real `Response`s.
- docs: `frame-ancestors` admits the proxy AND the Inspector page (every
  ancestor is checked); the shared helper falls back to loopback, not
  `'none'`; the showcase origin is `127.0.0.1`, the default bind address.
- tests: null/array body, post-listen error, and both factories receiving a
  working `publishAppDocument`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/README.md

@@ -160,7 +160,11 @@ Inspector owns no domain infrastructure — it cannot serve
 - The app document is served from there under an unguessable path, its
   per-app CSP delivered as a real response **header** (stronger than the
   `<meta>` the `srcdoc` path relies on) plus a `frame-ancestors` that admits
-  only the sandbox proxy.
+  only the two origins in its real ancestor chain — the sandbox proxy that
+  frames it, **and** the Inspector page that frames the proxy. Both are
+  required, not belt-and-braces: `frame-ancestors` is checked against every
+  ancestor, so omitting the Inspector's own origin blocks the frame outright
+  (see `appDocumentEmbedders`).
 - The inner iframe is granted `allow-same-origin` on this path **only** —
   that is what makes the origin real rather than opaque. It is not a
   weakening of #1565: the listener is on its own port, so the app is