feat(executor): expose ctx.trigger_depth and pin the invoke depth bump

`invoke_callee_sees_incremented_depth` asserted NOTHING — it ended `let _ = resp`
with a comment that `ctx.trigger_depth` wasn't exposed, so it would have passed
even if `invoke` forwarded the depth unchanged, i.e. never incrementing the
chain-depth counter that bounds runaway trigger loops.

`build_ctx_map` now surfaces `ctx.trigger_depth` (read-only: 0 for direct ingress,
+1 per synchronous `invoke` re-entry or dispatched handler) — the value the
author intended to read and a legitimate diagnostic for a script. The test now
pins the increment: the caller is depth 0, the invoke callee must see 1.

Mutation-verified: forwarding the depth unchanged makes the callee see 0 and the
test fails.

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

crates/executor-core/src/engine.rs

@@ -498,6 +498,14 @@ fn build_ctx_map(req: &ExecRequest) -> Map {
         "invocation_type".into(),
         invocation_type_str(req.invocation_type).into(),
     );
+    // Read-only depth of this call in a trigger/invoke chain: 0 for direct
+    // ingress, +1 per synchronous re-entry (`invoke`) or dispatched handler.
+    // Exposed so a script can observe where it sits in the chain (the same value
+    // the platform bounds via `trigger_depth_max`).
+    ctx.insert(
+        "trigger_depth".into(),
+        (i64::from(req.trigger_depth)).into(),
+    );
 
     let mut request = Map::new();
     request.insert("path".into(), req.path.clone().into());

crates/executor-core/tests/sdk_invoke.rs

@@ -194,16 +194,23 @@ async fn invoke_callee_sees_incremented_depth() {
         #{ statusCode: 200, body: d }
     "#
     .to_string();
+    // The caller is direct ingress (depth 0); the callee, re-entered via
+    // `invoke`, must see depth 1. Previously this test asserted NOTHING (`let _ =
+    // resp`) because `ctx.trigger_depth` wasn't exposed — so it would have passed
+    // even if `invoke` forwarded the depth unchanged (never incrementing the
+    // chain-depth counter that bounds runaway trigger loops). `ctx.trigger_depth`
+    // is now surfaced in build_ctx_map, so we can pin the increment.
     let req = baseline_request(app);
+    assert_eq!(req.trigger_depth, 0, "the caller is direct ingress");
     let resp = tokio::task::spawn_blocking(move || engine.execute(&src, req))
         .await
         .unwrap()
         .unwrap();
-    // ctx exposes trigger_depth? — not yet (it's not in build_ctx_map).
-    // Skip the strict assertion — the test still ensures the invoke
-    // chain didn't throw. (See HANDBACK §11 for cx.trigger_depth surface
-    // exposure as a v1.2 follow-up.)
-    let _ = resp;
+    assert_eq!(
+        resp.body,
+        json!(1),
+        "the invoke callee must see trigger_depth = caller + 1"
+    );
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]