feat(xlang): CLDK.workspace() multi-language composition

[rahlk]

CLDK.workspace(...): compose multiple language facades over one repo (or several) into a single queryable view. Fluent chains span facades; .reachable_to() traverses boundary edges with their confidence labels.

API sketch:

ws = CLDK.workspace({
    "frontend": CLDK.typescript(project_path="web/"),
    "api": CLDK.java(project_path="service/"),
    "worker": CLDK.python(project_path="jobs/"),
})
ws.callables().with_decorator("GetMapping").transitive_callers()  # spans languages

Part of #196. Branch: feat/issue-<n>.