Add Apple graph commands and queries

Summary

• Add typed frontend graph models for Apple targets, capabilities, providers, schemas, diagnostics, and typed manifest attrs.
• Adapt the first Apple schema surface from Bazel rules_apple and Buck2 Apple concepts, including headers, SDK frameworks and dylibs, resources, asset catalogs, Info.plist substitutions, entitlements, provisioning profiles, signing, test hosts, test plans, and privacy manifests.
• Move built-in Apple rule metadata into a Starlark prelude that lowers into Once’s typed Rust graph schemas while keeping once.toml as the user-authored manifest surface.
• Add once query targets, once query capabilities, once query schema, plus cache-backed graph once build, once run, and once test commands that materialize artifacts under .once/out/<target>/.
• Document the Apple graph model, adapted rule concepts, Starlark rule layer, and build, run, and test commands in docs/guide/apple-graph.md.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test -p once-frontend -p once-cli
• mise exec -- cargo clippy -p once-frontend -p once-cli --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- shellspec

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