Compile apple_library via Starlark rule implementation with caching

The apple_library target kind now compiles Swift code through a real Starlark rule implementation instead of a placeholder shell script. Once evaluates the prelude rule per target, builds an xcrun-driven swiftc command line, and produces a cacheable static library plus Swift module. Recursive builds work across apple_library dependencies, cache hits are reused when nothing changes, and source changes in a dependency invalidate downstream cache slots. Other Apple target kinds (framework, application, test bundle) continue to use their existing placeholder scripts while the new path is gated to finished rule implementations.