Declare targets in fabrik.star files and build them with fabrik run

Fabrik now evaluates per-package fabrik.star build files through a typed Starlark frontend. You can declare targets using built-in helpers like rust_binary, rust_library, and rust_test, use a glob() primitive that expands relative to the package directory, and reference targets by labels such as //pkg:name. fabrik targets lists every declared target in the workspace, and fabrik run //pkg:name builds or runs the target through the action cache. Ad-hoc command caching has moved to fabrik exec so the tool exposes one production verb for targets and one substrate escape hatch.