feat(graph/apple): multi-arch, swift_macro, .hmap, and select() for apple_library

Summary

• Build out the four “Not yet implemented” features the Apple graph guide previously listed: multi-arch + lipo fan-out, a swift_macro rule plus auto-loaded compiler plugins on apple_library, binary .hmap emission alongside the modulemap, and a manifest-level select() runtime for configurable = True attributes.
• Keep all toolchain knowledge in the Starlark prelude. Rust gains one new generic primitive, write_bytes(path, list[int]), used by the prelude to emit the Apple header-map binary; the format itself is constructed in Starlark. The hardcoded APPLE_RULES export name is now the generic RULES, and the executor docstring states explicitly that Rust has no knowledge of the build systems composed on top of its primitives.
• Apple graph docs updated to match: the rules list is split into “Implemented” (apple_library, swift_macro) and “Declared, awaiting implementation” (the bundle / app / test-host kinds), the agent workflows section is removed, and a Configurable attributes section documents the TOML select shape.

Testing

• cargo test -p once-frontend (60 passed)
• cargo check --workspace

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