feat(scoring): surface the merged-PR history floor in the score breakdown

src/services/score-breakdown.ts

@@ -99,6 +99,38 @@ function openIssueBreakdown(preview: ScorePreviewResult): ScoreMultiplierBreakdo
   };
 }
 
+// Sibling of openIssueBreakdown/openPrBreakdown for the merged-PR history floor (upstream MIN_VALID_MERGED_PRS):
+// a contributor whose observed merged-PR count on this repo is below the floor has the entire preview zeroed.
+// Explained here so a miner sees the same actionable breakdown the open-PR / open-issue gates already provide.
+function mergedHistoryBreakdown(preview: ScorePreviewResult): ScoreMultiplierBreakdown {
+  const { mergedHistoryMultiplier } = preview.scoreEstimate;
+  const { mergedPrFloor, mergedPullRequests } = preview.gates;
+  // mergedPullRequests is optional: when unobserved the floor is not enforced (the multiplier stays 1).
+  if (mergedPullRequests === undefined) {
+    return {
+      component: "mergedHistoryMultiplier",
+      band: "neutral",
+      summary: `Merged-PR history floor is not enforced for this preview (no contributor history observed; upstream floor is ${mergedPrFloor}).`,
+      lever: "No action needed for this preview; the upstream merged-PR floor applies once contributor history is observed.",
+      leverageScore: 0,
+    };
+  }
+  const band = bandForMultiplier(mergedHistoryMultiplier);
+  return {
+    component: "mergedHistoryMultiplier",
+    band,
+    summary:
+      mergedPullRequests >= mergedPrFloor
+        ? `Merged PR history (${mergedPullRequests}) meets the upstream floor (${mergedPrFloor}).`
+        : `Merged PR history (${mergedPullRequests}) is below the upstream floor (${mergedPrFloor}), so this preview is zeroed.`,
+    lever:
+      mergedPullRequests >= mergedPrFloor
+        ? "Keep landing merged PRs in this repo to maintain contributor history."
+        : "Land more merged PRs in this repo to clear the contributor-history floor before relying on this preview.",
+    leverageScore: mergedPullRequests >= mergedPrFloor ? 5 : 100,
+  };
+}
+
 function credibilityBreakdown(preview: ScorePreviewResult): ScoreMultiplierBreakdown {
   const { credibilityMultiplier } = preview.scoreEstimate;
   const { credibilityObserved, credibilityFloor } = preview.gates;
@@ -239,6 +271,7 @@ export function explainScoreBreakdown(preview: ScorePreviewResult): ScoreBreakdo
     reviewPenaltyBreakdown(preview),
     openPrBreakdown(preview),
     openIssueBreakdown(preview),
+    mergedHistoryBreakdown(preview),
   ].map((entry) => ({
     ...entry,
     summary: sanitizePublicComment(entry.summary),

test/unit/score-breakdown.test.ts

@@ -83,6 +83,7 @@ describe("explainScoreBreakdown", () => {
         "reviewPenaltyMultiplier",
         "openPrMultiplier",
         "openIssueMultiplier",
+        "mergedHistoryMultiplier",
       ]),
     );
     for (const component of breakdown.components) {
@@ -97,6 +98,28 @@ describe("explainScoreBreakdown", () => {
     expect(breakdown.components.find((entry) => entry.component === "openIssueMultiplier")).toMatchObject({ band: "full" });
   });
 
+  it("explains the merged-PR history floor as neutral (unobserved), full (meets floor), and blocked (below floor)", () => {
+    // Unobserved history -> floor not enforced -> neutral.
+    const unobserved = explainScoreBreakdown(
+      buildScorePreview({ repo, snapshot, input: { repoFullName: repo.fullName, contributorLogin: "miner", sourceTokenScore: 40, totalTokenScore: 60, sourceLines: 80, openPrCount: 1, credibility: 0.9, linkedIssueMode: "none" } }),
+    );
+    expect(unobserved.components.find((entry) => entry.component === "mergedHistoryMultiplier")).toMatchObject({ band: "neutral" });
+
+    // Observed >= upstream floor (MIN_VALID_MERGED_PRS = 3) -> full.
+    const meets = explainScoreBreakdown(
+      buildScorePreview({ repo, snapshot, input: { repoFullName: repo.fullName, contributorLogin: "miner", sourceTokenScore: 40, totalTokenScore: 60, sourceLines: 80, openPrCount: 1, credibility: 0.9, linkedIssueMode: "none", mergedPullRequests: 5 } }),
+    );
+    expect(meets.components.find((entry) => entry.component === "mergedHistoryMultiplier")).toMatchObject({ band: "full" });
+
+    // Observed < floor -> blocked, and the merged-PR lever is the top-leverage one.
+    const blocked = explainScoreBreakdown(
+      buildScorePreview({ repo, snapshot, input: { repoFullName: repo.fullName, contributorLogin: "miner", sourceTokenScore: 40, totalTokenScore: 60, sourceLines: 80, openPrCount: 1, credibility: 0.9, linkedIssueMode: "none", mergedPullRequests: 1 } }),
+    );
+    expect(blocked.components.find((entry) => entry.component === "mergedHistoryMultiplier")).toMatchObject({ band: "blocked", leverageScore: 100 });
+    expect(blocked.highestLeverageLever.lever).toMatch(/merge/i);
+    expect(JSON.stringify(blocked)).not.toMatch(FORBIDDEN);
+  });
+
   it("explains an over-threshold open-issue count as a blocked open-issue spam gate", () => {
     const preview = buildScorePreview({
       repo,