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

src/download/checksum/known-version.ts

@@ -0,0 +1,23 @@
+import * as semver from "semver";
+import { KNOWN_CHECKSUMS } from "./known-checksums";
+
+const VERSION_IN_CHECKSUM_KEY_PATTERN =
+  /-(\d+\.\d+\.\d+(?:[-+][0-9A-Za-z.-]+)?)$/;
+
+export function getLatestKnownVersion(): string {
+  const versions = new Set<string>();
+
+  for (const key of Object.keys(KNOWN_CHECKSUMS)) {
+    const version = key.match(VERSION_IN_CHECKSUM_KEY_PATTERN)?.[1];
+    if (version !== undefined) {
+      versions.add(version);
+    }
+  }
+
+  const latestVersion = [...versions].sort(semver.rcompare)[0];
+  if (!latestVersion) {
+    throw new Error("Could not determine latest known version from checksums.");
+  }
+
+  return latestVersion;
+}