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/checksum/known-version.test.ts

@@ -0,0 +1,20 @@
+import { expect, it, jest } from "@jest/globals";
+
+jest.unstable_mockModule(
+  "../../../src/download/checksum/known-checksums",
+  () => ({
+    KNOWN_CHECKSUMS: {
+      "aarch64-apple-darwin-1.9.0": "checksum",
+      "x86_64-unknown-linux-gnu-1.8.0": "checksum",
+      "x86_64-unknown-linux-gnu-1.10.0": "checksum",
+    },
+  }),
+);
+
+const { getLatestKnownVersion } = await import(
+  "../../../src/download/checksum/known-version"
+);
+
+it("returns the highest version with a built-in checksum", () => {
+  expect(getLatestKnownVersion()).toBe("1.10.0");
+});