test(suite): replace plan-label references with self-contained wording

Rust test comments/assert messages and deep-research script docstrings
cited numbered requirement labels (R3/R5) and task labels (TASK-016,
TASK-021) that are defined only in an external plan document. Reword
each to say what it means on its own: fail-safe crash guards, the
validator's fallback-capture warning, task-queue anchors.

assets/agents/deep-research/scripts/parse_request.py

@@ -10,7 +10,7 @@ Routing (`_next`):
   - prompt present -> (no _next; static next: plan)
   - prompt empty   -> ask_topic
 
-Fail-safe (R3): a crash here (e.g. malformed GRAPH_STATE) means the
+Fail-safe: a crash here (e.g. malformed GRAPH_STATE) means the
 caller's prompt cannot be read at all, so proceeding silently is
 meaningless — the degraded route is `ask_topic`, which asks the user for
 the topic directly, plus a PIPELINE-FAULT entry so the loss is visible in