fix(audit): require integer rolling retention keep limits

Validate keepPerFamily/--keep-per-family as non-negative integers, remove silent flooring, add regression coverage, and sync runbook/docs wording.

src/audit/phase0BaselineArtifactRetention.ts

@@ -142,11 +142,17 @@ export function planRollingPhase0ArtifactRetention(
   fileNames: string[],
   keepPerFamily: number,
 ): Phase0RollingArtifactRetentionPlan {
-  if (!Number.isFinite(keepPerFamily) || keepPerFamily < 0) {
+  if (!Number.isFinite(keepPerFamily)) {
+    throw new Error('keepPerFamily must be a finite integer greater than or equal to 0.');
+  }
+  if (!Number.isInteger(keepPerFamily)) {
+    throw new Error('keepPerFamily must be an integer greater than or equal to 0.');
+  }
+  if (keepPerFamily < 0) {
     throw new Error('keepPerFamily must be greater than or equal to 0.');
   }
 
-  const keepLimit = Math.floor(keepPerFamily);
+  const keepLimit = keepPerFamily;
   const parsed = collectRollingPhase0ArtifactFiles(fileNames);
 
   const keep: Phase0RollingArtifactFile[] = [];