fix(vercel-optimize): resolve root workspace package exports

Summary

Support root exports shorthand in the Vercel Optimize workspace resolver.

buildPackageLookup currently only iterates object entries as subpath keys. A package with "exports": "./index.js", a root fallback array, or a root conditional object such as { "import": "./index.js" } therefore returns null for its bare package import. As a result, resolveWorkspaceImports omits that package's source entry from the investigation's file list.

Normalize these root forms to a . entry before using the existing target selection logic. Explicit subpath maps and wildcard resolution remain unchanged; root-only exports do not expose private subpaths. This does not attempt to replace the existing conditional-target heuristic with a full Node resolver.

Reference: Node.js exports sugar.

Tests

• Three root-form regression cases, including assertions that an unexported subpath stays unresolved.
• A filesystem-backed integration case showing that a bare import reaches resolveWorkspaceImports.
• All four new cases fail before the runtime change and pass afterward.
• Full Vercel Optimize suite: 773 passed, 0 failed.
• Generated documentation freshness, citation checks (99 URLs, 13 rule references), and git diff --check pass.

Found by reviewing the resolver directly, rather than implementing an existing issue. AI-assisted implementation and validation; the reproducer uses local synthetic packages, not a claimed production incident.