feat(core-audit): MultiSinkAuditLog fan-out with settle-all + stderr fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-audit/src/multi-sink-audit-log.ts

@@ -0,0 +1,45 @@
+import type { AuditEntry } from "@repo/core-shared/audit";
+import type { IAuditLog } from "./audit-log.interface";
+
+/**
+ * Fan-out wrapper. Delivers each entry to every inner sink with settle-all
+ * semantics — one failing sink doesn't drop the audit entry from others.
+ *
+ * Failures emit a structured `audit-sink-error` JSON line to stderr.
+ * Stderr (not via OTel/Sentry) avoids recursion: if Sentry is one of the
+ * sinks failing and we routed the error back through Sentry's reporter,
+ * we'd loop. Stderr is consumed by the same log shipper as audit entries
+ * themselves, so the operator sees the failure in their aggregator.
+ */
+export class MultiSinkAuditLog implements IAuditLog {
+  constructor(private readonly sinks: IAuditLog[]) {}
+
+  async record(entry: AuditEntry): Promise<void> {
+    const results = await Promise.allSettled(this.sinks.map((s) => s.record(entry)));
+    for (const r of results) {
+      if (r.status === "rejected") {
+        this.reportSinkError(r.reason);
+      }
+    }
+  }
+
+  async eraseSubject(actorId: string, mode: "pseudonymize" | "delete"): Promise<void> {
+    const results = await Promise.allSettled(
+      this.sinks.map((s) => s.eraseSubject(actorId, mode)),
+    );
+    for (const r of results) {
+      if (r.status === "rejected") {
+        this.reportSinkError(r.reason);
+      }
+    }
+  }
+
+  private reportSinkError(reason: unknown): void {
+    const line = JSON.stringify({
+      _type: "audit-sink-error",
+      error: String(reason),
+      at: new Date().toISOString(),
+    });
+    process.stderr.write(line + "\n");
+  }
+}