feat(sdk): build library packages to .s2lib

buildLibrary() bundles a s2script.kind:"library" package into dist/<name>.s2lib
(manifest.json + index.js + index.d.ts only, types required, pluginDependencies/
publishes refused). buildPackage() dispatches plugin vs. library by kind and is
now the CLI's single entry point; buildPlugin refuses a library outright, naming
the fix.

Workspace-mode `s2s build` also now builds every ws.libs member declaring
s2script.kind:"library" to its own .s2lib (previously invisible: such a member
sits in the workspace's structural "everything that isn't a declared plugin"
bucket, not its s2script.workspace.plugins globs), ahead of the plugin loop for
a sensible log — a workspace sibling library is resolved from source by its
consumer's own build, so this ordering is not a dependency.