test(#1978): note remaining overload-coverage gap for follow-up

The four overload tests (Forum/Parallel/Loop/Branch) I attempted in the
previous commit didn't compile cleanly — each composition operator has
its own DSL shape and chasing all four for one batch wasn't worth it.
Reverted to the 19 tests that pass. Inline note documents the residual
gap and what closes it.

The serve$lambda$4/6/8/10 + corresponding `replaced int return with 0`
mutants on the four uncovered overloads survive this batch.

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

Author: Skobeltsyn

src/test/kotlin/agents_engine/runtime/LiveRunnerCliAndOnceTest.kt

@@ -243,4 +243,12 @@ class LiveRunnerCliAndOnceTest {
         val errExit = LiveRunner.serve(trivialAgent(), arrayOf("--nonsense")) { output = pw }
         assertNotEquals(0, errExit, "error path must NOT return 0 (happy-path exit code)")
     }
+
+    // ── Note: the four other serve() overloads (Forum/Parallel/Loop/Branch) ─
+    // ── have their own serve$lambda$4/6/8/10 + corresponding int-return    ─
+    // ── mutants that survive this batch. Killing them needs an --once test ─
+    // ── per overload — each composition operator's DSL is slightly         ─
+    // ── different (Forum uses participants + captain DSL; Loop's loop      ─
+    // ── factory varies by what's wrapped; Branch's branch DSL takes a      ─
+    // ── routing block). Left as a follow-up to keep this batch small.      ─
 }