feat(changelog): write down what changed, tied to git

Forty-four reader-facing entries and ten release notes, backfilled from
the history and the specs, each naming the commit or range it describes.
Tests pin the shape; a script checks the refs exist, since CI's shallow
checkout can't.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/changelog.test.ts

@@ -0,0 +1,65 @@
+import { describe, expect, test } from "bun:test";
+import { existsSync } from "node:fs";
+import {
+  compareUrl,
+  ENTRIES,
+  EntrySchema,
+  launchWindow,
+  newestId,
+  NOTES,
+  NoteSchema,
+  unseen,
+  type Entry,
+} from "../src/shared/changelog.ts";
+
+/**
+ * The changelog is reader-facing copy tied to git. These pin its shape — the
+ * refs' existence is `scripts/check-changelog.ts`'s job, since CI is shallow.
+ */
+describe("the changelog's content", () => {
+  test("every entry and note parses", () => {
+    for (const e of ENTRIES) expect(EntrySchema.safeParse(e).success).toBe(true);
+    for (const n of NOTES) expect(NoteSchema.safeParse(n).success).toBe(true);
+  });
+
+  test("ids are unique, and both lists are newest first", () => {
+    expect(new Set(ENTRIES.map((e) => e.id)).size).toBe(ENTRIES.length);
+    expect(new Set(NOTES.map((n) => n.id)).size).toBe(NOTES.length);
+    const sorted = (xs: { date: string }[]) =>
+      xs.every((x, i) => i === 0 || xs[i - 1]!.date >= x.date);
+    expect(sorted(ENTRIES)).toBe(true);
+    expect(sorted(NOTES)).toBe(true);
+  });
+
+  test("every entry's note exists, and every note's specs exist on disk", () => {
+    const ids = new Set(NOTES.map((n) => n.id));
+    for (const e of ENTRIES) if (e.note !== undefined) expect(ids).toContain(e.note);
+    for (const n of NOTES) for (const p of n.specs ?? []) expect(existsSync(p)).toBe(true);
+  });
+});
+
+describe("what a reader has not seen", () => {
+  const e = (id: string, date: string): Entry => ({
+    id, date, kind: "feature", title: id, ref: "abcdef1",
+  });
+  const list = [e("c", "2026-10-13"), e("b", "2026-10-05"), e("a", "2026-09-01")];
+
+  test("everything newer than the one they dismissed", () => {
+    expect(unseen(list, "b", "2026-10-13").map((x) => x.id)).toEqual(["c"]);
+    expect(unseen(list, "c", "2026-10-13")).toEqual([]);
+  });
+
+  test("never recorded: the last two weeks, once", () => {
+    expect(unseen(list, undefined, "2026-10-13").map((x) => x.id)).toEqual(["c", "b"]);
+  });
+
+  test("a recorded id that is gone falls back to the launch window, not the whole history", () => {
+    expect(unseen(list, "renamed", "2026-10-13").map((x) => x.id)).toEqual(["c", "b"]);
+  });
+
+  test("helpers", () => {
+    expect(launchWindow(list, "2026-10-13", 3).map((x) => x.id)).toEqual(["c"]);
+    expect(newestId(list)).toBe("c");
+    expect(compareUrl("https://r", "a1b2c3d", "e4f5a6b")).toBe("https://r/compare/a1b2c3d...e4f5a6b");
+  });
+});