feat(executor-core): plumb app_id/principal/depth through ExecRequest

Adds the four internal-only fields every v1.1.x stateful service needs
to isolate by app and audit by caller:

  - app_id            — owning app for this invocation
  - principal         — Option<Principal>; data-plane is unauthenticated
                        today so the orchestrator passes None until the
                        opportunistic middleware lands in the next commit
  - trigger_depth     — 0 for direct invocations; the triggers framework
                        (v1.1.1) bounds runaway feedback loops via this
  - root_execution_id — equal to execution_id for direct invocations;
                        preserved across trigger fan-out for audit grouping

ExecRequest stays serializable (cluster mode still has to ship it across
processes when v1.3+ arrives). principal is `#[serde(skip)]` because
shared::Principal has no wire derivation today — when cluster mode lands
the wire-Principal question gets revisited properly.

Engine now carries a Services bundle (empty in v1.1.0). Engine::execute
constructs an SdkCallCx from the request and hands it to sdk::register_all
just after the per-call Rhai engine is built. The hook is a no-op in v1.1.0;
v1.1.1 KV registers its first native fns there.

Adds ExecError::Overloaded { retry_after_secs } and the matching 503 +
Retry-After mapping in orchestrator-core's IntoResponse. The gate that
actually produces this variant lands in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/executor-core/tests/sdk_contract.rs

@@ -23,7 +23,7 @@
 use std::collections::BTreeMap;
 
 use picloud_executor_core::{Engine, ExecRequest, InvocationType, Limits, LogLevel};
-use picloud_shared::{ExecutionId, RequestId, ScriptId, ScriptSandbox};
+use picloud_shared::{AppId, ExecutionId, RequestId, ScriptId, ScriptSandbox, Services};
 use serde_json::{json, Value};
 
 // ----------------------------------------------------------------------------
@@ -31,12 +31,13 @@ use serde_json::{json, Value};
 // ----------------------------------------------------------------------------
 
 fn engine() -> Engine {
-    Engine::new(Limits::default())
+    Engine::new(Limits::default(), Services::new())
 }
 
 fn baseline_request() -> ExecRequest {
+    let execution_id = ExecutionId::new();
     ExecRequest {
-        execution_id: ExecutionId::new(),
+        execution_id,
         request_id: RequestId::new(),
         script_id: ScriptId::new(),
         script_name: "contract".into(),
@@ -48,6 +49,10 @@ fn baseline_request() -> ExecRequest {
         query: BTreeMap::new(),
         rest: String::new(),
         sandbox_overrides: ScriptSandbox::default(),
+        app_id: AppId::new(),
+        principal: None,
+        trigger_depth: 0,
+        root_execution_id: execution_id,
     }
 }