feat(#50): make the four 0-100 stats consequential

- Add a hard military conquest gate (canConquer, CONQUEST_MILITARY_MARGIN=15)
  enforced in applyTurnResult: a contested invasion is repelled when the
  attacker's military is more than 15 below the defender's; re-claims of
  unowned regions are never gated. Emits a new 'repelled' event.
- Document the soft levers in the system prompt: stability<30 + gathering
  marker -> non-military takeover; economy growth/decay drives other stats.
- Emit bounded frontierMilitary (per-neighbour balance) and takeoverRisks
  hints in the user payload (only when present).
- repelled outcome color (#9f1239); full unit test coverage; stub untouched.
- Bump to v0.30.0.

Fixes #50

shared/protocol.ts

@@ -452,6 +452,42 @@ export function nationColor(id: string): string {
   return `hsl(${hue} 58% 46%)`;
 }
 
+// --- Combat: the military hard gate (v0.30, #50) ------------------------------
+//
+// The one rule the engine enforces on the player's behalf instead of trusting
+// the LLM: a contested conquest only succeeds when the attacker is not MORE
+// than `CONQUEST_MILITARY_MARGIN` below the defender. The LLM still proposes
+// the transfer; the engine decides whether it holds (see `applyTurnResult`).
+// Re-claims of unowned / no-man's-land regions are never gated.
+
+/**
+ * (v0.30, #50) The margin of military superiority an attacker must NOT be more
+ * than below the defender for a contested conquest to succeed. A gap of exactly
+ * this many points is still enough to win; a gap one larger is repelled.
+ */
+export const CONQUEST_MILITARY_MARGIN = 15;
+
+/**
+ * (v0.30, #50) Hard military gate for a **contested** conquest (a region
+ * currently owned by a different nation). Returns `true` — the invasion
+ * proceeds — unless the attacker's military is MORE than `margin` below the
+ * defender's: equal, or within `margin` (default {@link CONQUEST_MILITARY_MARGIN}),
+ * is enough to win, while a gap larger than `margin` is repelled.
+ *
+ * The comparison is intentionally one-sided (it only protects the *defender*
+ * from an overwhelming odds gap, not the attacker from a small disadvantage):
+ * `attacker < defender - margin` → repel; everything else (equal, within the
+ * margin, or ahead) → proceed. Pure + shared so the engine, the tests, and the
+ * prompt's frontier-balance hint all agree on the outcome.
+ */
+export function canConquer(
+  attackerMil: number,
+  defenderMil: number,
+  margin: number = CONQUEST_MILITARY_MARGIN,
+): boolean {
+  return !(attackerMil < defenderMil - margin);
+}
+
 // --- No man's land (v0.22, #16 P3) — the map presentation contract ---
 //
 // An unowned region has two distinct looks, and the difference is NOT in the

shared/types.ts

@@ -79,7 +79,15 @@ export const EVENT_TYPES: EventType[] = [
   'cultural',
 ];
 
-export type EventOutcome = 'success' | 'failure' | 'partial' | 'neutral';
+/**
+ * The outcome of a narrative event. `success` / `failure` / `partial` /
+ * `neutral` are the four the LLM is asked to emit. `repelled` (v0.30, #50) is
+ * reserved for the engine: it is emitted automatically when the military hard
+ * gate rejects a contested conquest (an invasion whose attacker was more than
+ * `CONQUEST_MILITARY_MARGIN` below the defender) — the LLM never produces it,
+ * and the client renders it in a distinct muted-red failure tone.
+ */
+export type EventOutcome = 'success' | 'failure' | 'partial' | 'neutral' | 'repelled';
 
 // --- Map markers (v0.27, #44) -------------------------------------------------
 //