feat: reconcile split-surface CORS

Authored-By: OpenAI (GPT-5) <noreply@openai.com>

README.md

@@ -52,6 +52,7 @@ artifacts:
   - name: distributions
     type: static
     content_dir: dist
+    cors_origins: ['*']
     publish:
       bucket: baseline-dist
       credentials:
@@ -141,6 +142,11 @@ protected input from entering a public artifact through dereference. Split
 storage endpoints are pinned to Garage, and each website
 authority is derived from its bucket; a site cannot expose an arbitrary backend.
 
+`cors_origins` is reconciled as a bucket policy on every split publication. Values are either `*`
+or HTTPS origins; browser access is limited to `GET` and `HEAD`. Omitting the field removes stale
+CORS from that bucket. Protected artifacts cannot allow wildcard CORS. All immutable objects and
+all bucket CORS policies complete before mutable channels change.
+
 Each split route gets a bucket-specific `<bucket>.web.sjc001.fritzlab.net`
 ExternalName Service annotated to disable pass-host-header and a separate Ingress. Route
 Ingresses share the hostname's certificate Secret. The access middleware and