Emdash source with visual editor image upload fix

Fixes:
1. media.ts: wrap placeholder generation in try-catch
2. toolbar.ts: check r.ok, display error message in popover

packages/plugins/marketplace-test/README.md

@@ -0,0 +1,29 @@
+# Marketplace Test Plugin
+
+End-to-end test plugin for the EmDash marketplace publish and audit pipeline.
+
+## What it does
+
+- Hooks into `content:beforeSave` to log save events
+- Exposes a `/ping` route and an `/events` route
+- Declares `read:content` and `write:content` capabilities
+- Includes icon and screenshot assets for image audit testing
+
+## Usage
+
+Bundle and publish to a marketplace instance:
+
+```bash
+emdash plugin bundle --dir packages/plugins/marketplace-test
+emdash plugin publish dist/marketplace-test-0.1.0.tar.gz --registry https://emdash-marketplace.cto.cloudflare.dev
+```
+
+## Testing
+
+This plugin is designed to exercise every step of the marketplace pipeline:
+
+1. **Bundle** — `emdash plugin bundle` builds `backend.js` from `sandbox-entry.ts`
+2. **Upload** — tarball includes manifest, backend, icon, screenshot, README
+3. **Code audit** — Workers AI analyzes `backend.js` (should pass — clean code)
+4. **Image audit** — Workers AI analyzes `icon.png` and `screenshots/` (should pass)
+5. **Status resolution** — enforcement mode determines final status