feat(modules): GroupKvService + kv::shared SDK handle (§11.6 C3)

The runtime for shared group collections. A script reaches a shared store via
the explicit kv::shared("name") handle (distinct GroupKvHandle Rhai type, so
private-vs-shared scope is a deliberate choice). The service resolves the
owning group from cx.app_id's ancestor chain — the script never names a group,
and that walk is the isolation boundary (a foreign app's chain never contains
the owning group → CollectionNotShared).

- shared: GroupKvService trait + GroupKvError + NoopGroupKvService; threaded
  into the Services bundle as a field defaulted to noop, opted into via a new
  with_group_kv() (keeps the ~14 Services::new call sites unchanged).
- manager-core: GroupKvServiceImpl with the reads-open/writes-authed split —
  reads use script_gate (anonymous public scripts skip), writes use the new
  script_gate_require_principal (anonymous fails closed). Owner resolution
  behind a GroupCollectionResolver trait (Postgres impl + injectable fake);
  unit tests cover off-chain CollectionNotShared, anonymous-read-OK/
  anonymous-write-Forbidden, authed-no-role-Forbidden.
- executor-core: GroupKvHandle + kv::shared constructor + get/set/has/delete/
  list (Rhai dispatches by receiver type, reusing the block_on bridge).
- picloud: wire PostgresGroupKvRepo + resolver + GroupKvServiceImpl.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/authz.rs

@@ -457,6 +457,34 @@ pub async fn script_gate<E>(
     }
 }
 
+/// Like [`script_gate`], but **fails closed on an anonymous principal**: a
+/// script with `cx.principal == None` is rejected with `forbidden()` rather
+/// than skipped. Used for actions that must always be performed by an
+/// authenticated caller even though the surrounding service skips authz for
+/// public scripts — e.g. §11.6 group-collection WRITES (reads stay open via
+/// `script_gate`, writes require an authenticated editor+ on the owning group).
+///
+/// # Errors
+///
+/// Returns `forbidden()` when the principal is absent or the capability is
+/// denied, or `backend(repo_err.to_string())` on a repo error.
+pub async fn script_gate_require_principal<E>(
+    repo: &dyn AuthzRepo,
+    cx: &picloud_shared::SdkCallCx,
+    cap: Capability,
+    forbidden: impl FnOnce() -> E,
+    backend: impl FnOnce(String) -> E,
+) -> Result<(), E> {
+    let Some(principal) = cx.principal.as_ref() else {
+        return Err(forbidden());
+    };
+    match require(repo, principal, cap).await {
+        Ok(()) => Ok(()),
+        Err(AuthzDenied::Denied) => Err(forbidden()),
+        Err(AuthzDenied::Repo(e)) => Err(backend(e.to_string())),
+    }
+}
+
 // ----------------------------------------------------------------------------
 // Layer 1: role-derived grant
 // ----------------------------------------------------------------------------