feat(executor): thread the defining node into ExecRequest (Phase 4b C2)

Carry the resolved script's owner (its defining node) from dispatch into
the executor so `import` resolution can be lexical (§5.5). The executing
app (`app_id`) stays the SDK isolation boundary; `script_owner` is a
separate axis — the lexical origin for imports.

- `ExecRequest.script_owner: Option<ScriptOwner>` (serde default; `None`
  falls back to `App(app_id)` in the engine for old payloads / cluster wire).
- `ScriptOwner` gains `Serialize`/`Deserialize` for the wire.
- The engine computes `default_origin` and hands it to the resolver
  (used fully in C3; the resolve() lookup already uses it).
- Every dispatch site sets it from the resolved `Script.owner()`:
  the 4 dispatcher arms (queue/trigger/http/invoke_async, via a new
  `ResolvedTrigger.script_owner`), the orchestrator id-bypass, and the
  `invoke()` SDK path (new `ResolvedScript.owner`, so a group script
  invoked by an app resolves imports from the group).

Behaviour-preserving: app scripts still resolve `App(app_id)`; group
scripts can't yet carry imports (lifted in C4).

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

crates/executor-core/src/engine.rs

@@ -300,11 +300,18 @@ impl Engine {
         // installed with the real per-call resolver. The resolver owns
         // `cx.clone()` so cross-app isolation derives from this exact
         // call's context, not from any script-passed argument.
+        // The lexical origin for `import` resolution (§5.5): the top-level
+        // script's defining node. Falls back to the executing app when the
+        // request predates Phase 4b (old payloads / cluster wire).
+        let default_origin = req
+            .script_owner
+            .unwrap_or(picloud_shared::ScriptOwner::App(req.app_id));
         let resolver = PicloudModuleResolver::new(
             self.services.modules.clone(),
             cx.clone(),
             self.module_cache.clone(),
             effective_limits.module_import_depth_max,
+            default_origin,
         );
         engine.set_module_resolver(resolver);
         let self_engine = self.self_arc();