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/version/resolve.ts

@@ -2,6 +2,7 @@ import * as core from "@actions/core";
 import * as tc from "@actions/tool-cache";
 import * as pep440 from "@renovatebot/pep440";
 import * as semver from "semver";
+import { getLatestKnownVersion } from "../download/checksum/known-version";
 import {
   getAllVersions,
   getFirstMatchingVersion,
@@ -143,6 +144,10 @@ export async function resolveVersion(
 ): Promise<string> {
   core.debug(`Resolving version: ${versionInput}`);
 
+  if (versionInput.trim() === "latest-known") {
+    return getLatestKnownVersion();
+  }
+
   const context: ConcreteVersionResolutionContext = {
     manifestUrl,
     parsedSpecifier: parseVersionSpecifier(versionInput),