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/shared/src/script.rs

@@ -152,7 +152,7 @@ pub struct Script {
 /// a `CHECK ((group_id IS NULL) <> (app_id IS NULL))`; this enum is the
 /// in-memory witness of that invariant so call sites can `match` exhaustively
 /// instead of juggling two `Option`s.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub enum ScriptOwner {
     App(AppId),
     Group(GroupId),