Persist PR diff ranges during init

We don't use them yet and will re-save them during analysis.

src/init-action.ts

@@ -34,6 +34,11 @@ import {
   logUnwrittenDiagnostics,
   makeDiagnostic,
 } from "./diagnostics";
+import {
+  getPullRequestEditedDiffRanges,
+  writeDiffRangesJsonFile,
+  getDiffInformedAnalysisBranches,
+} from "./diff-informed-analysis-utils";
 import { EnvVar } from "./environment";
 import { Feature, Features } from "./feature-flags";
 import { loadPropertiesFromApi } from "./feature-flags/properties";
@@ -46,7 +51,7 @@ import {
   runDatabaseInitCluster,
 } from "./init";
 import { KnownLanguage } from "./languages";
-import { getActionsLogger, Logger } from "./logging";
+import { getActionsLogger, Logger, withGroupAsync } from "./logging";
 import {
   downloadOverlayBaseDatabaseFromCache,
   OverlayBaseDatabaseDownloadStats,
@@ -358,6 +363,7 @@ async function run() {
     });
 
     await checkInstallPython311(config.languages, codeql);
+    await computeAndPersistDiffRanges(codeql, features, logger);
   } catch (unwrappedError) {
     const error = wrapError(unwrappedError);
     core.setFailed(error.message);
@@ -770,6 +776,43 @@ async function run() {
   );
 }
 
+/**
+ * Compute and persist diff ranges when diff-informed analysis
+ * is enabled (feature flag + PR context). This writes the standard pr-diff-range.json
+ * file for later reuse in the analyze step. Failures are logged but non-fatal.
+ */
+async function computeAndPersistDiffRanges(
+  codeql: CodeQL,
+  features: Features,
+  logger: Logger,
+): Promise<void> {
+  try {
+    await withGroupAsync("Compute PR diff ranges", async () => {
+      const branches = await getDiffInformedAnalysisBranches(
+        codeql,
+        features,
+        logger,
+      );
+      if (!branches) {
+        return;
+      }
+      const ranges = await getPullRequestEditedDiffRanges(branches, logger);
+      if (ranges === undefined) {
+        return;
+      }
+      writeDiffRangesJsonFile(logger, ranges);
+      const distinctFiles = new Set(ranges.map((r) => r.path)).size;
+      logger.info(
+        `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s).`,
+      );
+    });
+  } catch (e) {
+    logger.warning(
+      `Failed to compute and persist PR diff ranges: ${getErrorMessage(e)}`,
+    );
+  }
+}
+
 function getTrapCachingEnabled(): boolean {
   // If the workflow specified something always respect that
   const trapCaching = getOptionalInput("trap-caching");