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/sdk/invoke.rs

@@ -179,6 +179,10 @@ fn invoke_blocking(
         rest: String::new(),
         sandbox_overrides: picloud_shared::ScriptSandbox::default(),
         app_id: cx.app_id,
+        // Lexical origin (§5.5): the callee's defining node — a group
+        // script's imports resolve from the group even when invoked by an
+        // app. `None` falls back to `App(cx.app_id)` in the engine.
+        script_owner: resolved.owner,
         // Same-app invoke is a function call, not a re-auth boundary —
         // inherit the caller's principal.
         principal: cx.principal.clone(),