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/orchestrator-core/src/api.rs

@@ -135,6 +135,7 @@ where
 
     let mut req = build_exec_request(id, &script.name, &headers, &body, app_id, principal)?;
     req.sandbox_overrides = script.sandbox;
+    req.script_owner = script.owner();
     let request_id = req.request_id;
     let request_path = req.path.clone();
     let request_headers = req.headers.clone();
@@ -628,6 +629,10 @@ fn build_exec_request(
         // Overwritten by the handler after the script is resolved.
         sandbox_overrides: picloud_shared::ScriptSandbox::default(),
         app_id,
+        // Set by the handler from the resolved script's owner. The
+        // id-bypass runs app-owned scripts only (group scripts 404 here),
+        // so this is always `App(app_id)`; `None` falls back to that.
+        script_owner: None,
         principal,
         // Direct invocations are at depth 0 with a self-referential
         // root. The triggers framework (v1.1.1) increments depth and