apple: an in-repo Swift iOS app builds and tests on a simulator (part of #1854) #6597

A Swift iOS app builds and runs its XCTests on an iOS simulator from rules_dx's own workspace, with no nested Bazel.

Plan (agent): size M, opus
Decision: use an in-repo fixture package built by the outer Bazel on macos_arm64. Register the Xcode C++ toolchain as a dev_dependency. Rejected: keep the nested //apple/tests:macos_swift_consumer_test. It has never passed (run 40614493224), it is slow, and #5291/#5757 remove nested apple tests.
Cause: no in-repo target builds an ios_application on macOS. rules_dx registers no Apple C++ toolchain (MODULE.bazel:490 has only @llvm). Consumers add it themselves (apple/README.md:248).
Change:

1. MODULE.bazel:490: after @llvm, add apple_cc_configure_extension from @apple_support//crosstool:setup.bzl as dev_dependency = True and register @local_config_apple_cc_toolchains//:all as dev. Check with --toolchain_resolution_debug that macOS targets keep @llvm and iOS simulator targets get the Apple toolchain. If @llvm claims iOS, register only the iOS toolchain targets from that repo, before @llvm.
2. New apple/tests/swift_app/BUILD.bazel with sources, all macos arm64 only:
   • a Rust rust_static_library with a C header cc_library and a swift_interop_hint
   • core swift_library and @main entry swift_library from //swift/rules:defs.bzl
   • apple_ios_app(entry=..., library=..., platforms=["simulator/arm64"])
   • apple_ios_app_test(deps=[testonly XCTest swift_library])
   • a device/arm64 apple_ios_app with a checked-in test .mobileprovision, behind a build_test
     Port the Swift sources from apple/tests/macos_swift_consumer_test.rs.
3. Same package: an sh_test that unzips the simulator .ipa and checks CFBundleIdentifier, MinimumOSVersion, DTPlatformName=iphonesimulator, lipo -archs arm64, otool -l platform 7, and the _ledger_total symbol.
4. Same package: failure_test negatives on the *_selection targets, macOS only: xcode_version = "9.9" (message "selects Xcode 9.9 but Bazel resolved") and device without provisioning_profile.
   Tests: on Linux, tools/dx test //apple/... //swift/... (the new targets skip as incompatible). Then evidence.yml with platforms ["macos_arm64"], scope //apple/tests/swift_app/... //apple/ios/..., test_timeout 1800. Attach executor.txt and the XCTest case names with nonzero executed counts.
   Host: macos (evidence.yml macos_arm64)
   Upstream: none expected.

Do after #6596
Part of #1854