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/server/app-origin-controller.ts

@@ -273,8 +273,15 @@ export function createAppOriginController(
             console.error("App origin server error:", err);
           }
           // Degrade rather than reject: `publish` then returns null and the
-          // renderer keeps working on the default srcdoc path.
+          // renderer keeps working on the default srcdoc path. Clearing
+          // `origin` is what makes that true — `publish` gates on it, not on
+          // `server`, so an error arriving AFTER a successful listen would
+          // otherwise keep minting URLs on a dead listener and the browser
+          // would never take the promised fallback. Dropping the documents
+          // with it releases their memory, since nothing can fetch them now.
           server = null;
+          origin = null;
+          documents.clear();
           settle({ port: 0, url: "" });
         });