GEN-1: add lethality rules + warning codes (SpSp lethal+renormalize, ReRe semi)

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

gerbil-manager-web/src/genetics/warnings.ts

@@ -0,0 +1,16 @@
+/**
+ * Warning codes emitted by the genetics engine. The engine is language-neutral:
+ * it emits CODES only. German texts are mapped in de.ts (UI layer).
+ */
+export enum GeneticsWarningCode {
+  /** Schecke × Schecke: SpSp embryos die in utero; ~25% fewer live young. */
+  ScheckeLethal = 'SCHECKE_LETHAL',
+  /** Rex × Rex: ReRe is semi-lethal; reduced viability of homozygous young. */
+  RexSemiLethal = 'REX_SEMI_LETHAL',
+}
+
+export interface GeneticsWarning {
+  readonly code: GeneticsWarningCode
+  /** Optional structured detail for the UI (e.g. fraction of young lost). */
+  readonly detail?: Record<string, string | number>
+}