Fix Grumman stack placement cancellation and inventory validation

docs/NEXUS_CARD_RULES.md

@@ -55,6 +55,8 @@ Source for the entire table: [the twelve original printed components](https://bo
 
 ## Effect integration boundaries
 
+Later focused source contracts cover [Emperor strength, purchases and revival](NEXUS_EMPEROR_RULES.md), [Ixian purchased-card replacement](NEXUS_IXIAN_REPLACEMENT_RULES.md), and [Fremen worm and revival effects](NEXUS_FREMEN_RULES.md). These audits distinguish implemented work from remaining source/timing questions; the current runtime evidence is tracked in [implementation status](IMPLEMENTATION_STATUS.md).
+
 **Atreides is the first complete effect family to implement.** It has independent rulebook and physical-face evidence, and the engine already has private plan-element inspection. The Nexus effect needs its own cause and receipt rather than temporarily changing the inspecting faction. The second element must differ from the already inspected element. Preserve the original disclosed value if an allowed later battle edit occurs; follow the existing plan commitment rules instead of refreshing an old observation. The card specifies an element, not the entire hand or battle plan. A weapon/defense request uses actual played-card classification, including the published Reinforcements/Harass exclusions, rather than leaking a card merely because it occupies that slot.
 
 **Cancellation needs the original attempted effect.** Betrayal is not a blanket Karama flag. For Atreides, identify the inspection attempt being prevented; for Voice, the command; for a traitor/Face Dancer, the actual revealed identity; for economy, the original invoice and settlement. Do not refund or replay a completed payment merely because the next input happens to be a Nexus play. Effects with no printed phase still need a safe explicit action boundary and cannot bypass another player's committed private choice.