Add latest-known version selector (#993)

## Summary

- add `latest-known` as an explicit version selector
- resolve it locally to the newest version in the bundled checksum table
- preserve existing default and `latest` behavior
- document custom-manifest checksum semantics and update published
bundles

## Testing

- `npm ci --ignore-scripts`
- `npm run all` (99 tests passed)

Closes #919

Refs: pi-session 019fed0e-6019-7504-911b-bd9955cbbd49

__tests__/download/download-version.test.ts

@@ -55,6 +55,12 @@ jest.unstable_mockModule("../../src/download/checksum/checksum", () => ({
   validateChecksum: mockValidateChecksum,
 }));
 
+const mockGetLatestKnownVersion = jest.fn(() => "0.9.25");
+
+jest.unstable_mockModule("../../src/download/checksum/known-version", () => ({
+  getLatestKnownVersion: mockGetLatestKnownVersion,
+}));
+
 const { downloadVersion, resolveVersion, rewriteToMirror } = await import(
   "../../src/download/download-version"
 );
@@ -72,6 +78,7 @@ describe("download-version", () => {
     mockGetFirstMatchingVersion.mockReset();
     mockGetArtifact.mockReset();
     mockValidateChecksum.mockReset();
+    mockGetLatestKnownVersion.mockClear();
 
     mockDownloadTool.mockResolvedValue("/tmp/downloaded");
     mockExtractTar.mockResolvedValue("/tmp/extracted");
@@ -90,6 +97,16 @@ describe("download-version", () => {
       expect(mockGetLatestVersion).toHaveBeenCalledWith(undefined);
     });
 
+    it("resolves latest-known without reading the manifest", async () => {
+      const version = await resolveVersion("latest-known", undefined);
+
+      expect(version).toBe("0.9.25");
+      expect(mockGetLatestKnownVersion).toHaveBeenCalledTimes(1);
+      expect(mockGetLatestVersion).not.toHaveBeenCalled();
+      expect(mockGetAllVersions).not.toHaveBeenCalled();
+      expect(mockGetFirstMatchingVersion).not.toHaveBeenCalled();
+    });
+
     it("stops at the first matching version in the default manifest", async () => {
       mockGetFirstMatchingVersion.mockImplementation(
         (predicate: (version: string) => boolean) =>