lean-rs v0.1.12

Shims-only host sessions

lean-rs-host now exposes LeanHost::load_shims_only(), a public bootstrap path for hosts that only need the bundled
Meta, elaboration, kernel, declaration, source-range, and info-tree services. It loads the bundled interop and
LeanRsHostShims dylibs, resolves the existing session symbols from the shim library, and deliberately skips opening
the user's :shared dylib. Sessions still import modules from the project's .olean search path; ad-hoc
LeanSession::call_capability calls return lean_rs.unsupported because no user library is attached.

Worker boundary: shims-only host handle

lean-rs-worker-parent now has LeanWorkerHostHandleBuilder::shims_only(...) and LeanWorkerHostHandle for worker
sessions backed only by bundled host shims. This keeps the existing LeanWorkerCapabilityBuilder contract strict for
user @[export] dylibs while giving downstream tools a path that does not run lake build <lib>:shared before opening
a session. The worker protocol adds HostSessionMode::{Capability, ShimsOnly} so the child can route shims-only opens
to LeanHost::load_shims_only().