Stop a second copy of the checkup due line from appearing

Two screens once carried their own copy of the same date arithmetic and quietly disagreed for a year. This fails the moment a second one is written, whatever it is named, and says where the real one lives.

src/__tests__/vorsorge-due-bucket-single-home-guard.test.ts

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+/**
+ * Structural guard on the checkup due-day bucket.
+ *
+ * Turning a reminder's `nextDueAt` into "today" / "tomorrow" / "in N days" /
+ * "overdue by N days" is calendar arithmetic, and it is easy to get wrong in a
+ * way that reads fine most of the time: subtract the two instants, divide by
+ * twenty-four hours, round. That answer disagrees with the wall calendar
+ * whenever the two times of day are more than twelve hours apart, which is
+ * most evenings.
+ *
+ * It was written that way once, fixed on the checkups page, and left unfixed
+ * in the hand-copied second body on the dashboard card. For a year the two
+ * screens gave different answers about the same appointment. This file exists
+ * so a third body cannot appear.
+ *
+ * These are tripwires, not proofs. Whether the bucketing is CORRECT is settled
+ * by `src/lib/measurement-reminders/__tests__/due-day.test.ts`. What this file
+ * settles is that there is only one of it.
+ */
+import { describe, it, expect } from "vitest";
+import { readFileSync, globSync } from "node:fs";
+import { join, sep } from "node:path";
+
+const SRC = join(process.cwd(), "src");
+
+/** The one home. Everything else imports from here. */
+const HOME = "lib/measurement-reminders/due-day.ts";
+
+/** How a consumer must reach it. */
+const HOME_IMPORT = 'from "@/lib/measurement-reminders/due-day"';
+
+/**
+ * The two screens that render the due line. Named here so this guard can never
+ * pass by having nothing left to check: if the surfaces stop importing the
+ * shared body, that is the regression, not a reason to relax the test.
+ */
+const RENDERING_SURFACES = [
+  "components/measurement-reminders/vorsorge-section.tsx",
+  "components/measurement-reminders/vorsorge-dashboard-card.tsx",
+];
+
+/**
+ * The keys only the bucket itself mints. A surface may COMPARE against the
+ * today / overdue keys to colour its badge, and both do — so those two are
+ * deliberately not listed. Nothing but the bucket ever needs to name
+ * "tomorrow", "in N days" or "none", so a file outside the home that contains
+ * one is a second bucket wearing a different function name.
+ */
+const MINTED_ONLY_BY_THE_BUCKET = [
+  "measurementReminders.nextDue.tomorrow",
+  "measurementReminders.nextDue.inDays",
+  "measurementReminders.nextDue.none",
+];
+
+const FIX_IT = [
+  `The due-day bucket lives in src/${HOME} and nowhere else.`,
+  `Import { relativeDueKey } ${HOME_IMPORT} instead of writing a second one.`,
+  "If the behaviour needs to change, change it there — both the checkups page",
+  "and the dashboard card read that body, and they must agree.",
+].join(" ");
+
+function sourceFiles(): string[] {
+  return globSync("**/*.{ts,tsx}", { cwd: SRC })
+    .map((p) => p.split(sep).join("/"))
+    .filter((p) => !p.startsWith("generated/"))
+    .filter((p) => !p.includes("__tests__/"))
+    .sort();
+}
+
+function read(rel: string): string {
+  return readFileSync(join(SRC, rel), "utf8");
+}
+
+/** A line that declares `relativeDueKey`, ignoring comment prose. */
+function declaresTheBucket(source: string): boolean {
+  return source
+    .split("\n")
+    .some(
+      (line) =>
+        /\b(function|const|let|var)\s+relativeDueKey\b/.test(line) &&
+        !/^\s*(\*|\/\/|\/\*)/.test(line),
+    );
+}
+
+describe("the checkup due-day bucket has one home", () => {
+  it("is declared in exactly one file", () => {
+    const declaring = sourceFiles().filter((rel) =>
+      declaresTheBucket(read(rel)),
+    );
+
+    expect(declaring, FIX_IT).toEqual([HOME]);
+  });
+
+  it("is reached by import everywhere it is used", () => {
+    const localUsers = sourceFiles()
+      .filter((rel) => rel !== HOME)
+      .filter((rel) => read(rel).includes("relativeDueKey"))
+      .filter((rel) => !read(rel).includes(HOME_IMPORT));
+
+    expect(localUsers, FIX_IT).toEqual([]);
+  });
+
+  it("is the only place the tomorrow, in-N-days and none keys are minted", () => {
+    // Catches a duplicate that was renamed on the way in: whatever it is
+    // called, a second bucket has to name the same keys to say the same words.
+    const minters = sourceFiles()
+      .filter((rel) => rel !== HOME)
+      .filter((rel) => {
+        const source = read(rel);
+        return MINTED_ONLY_BY_THE_BUCKET.some((key) => source.includes(key));
+      });
+
+    expect(minters, FIX_IT).toEqual([]);
+  });
+
+  it("is what both checkup screens read their due line from", () => {
+    for (const rel of RENDERING_SURFACES) {
+      expect(read(rel), `${rel} no longer reads the shared due line`).toContain(
+        HOME_IMPORT,
+      );
+    }
+  });
+});