[Swift] v0.1.0

First-class hermetic Swift build rules for Please

Added

• swift_library: Compiles Swift sources into static libraries (.a), Swift module interfaces (.swiftmodule), and metadata manifests (swift_metadata.json).
• swift_binary: Compiles standalone executables with -static-stdlib support on Linux.
• swift_test: Modern testing with swift-testing (import Testing, @Test, #expect), automatic async runner synthesis, JUnit XML reporting, and Cobertura line coverage.
• swift_toolchain: Hermetic toolchain configuration for Swift 6.3.3.
• tools/please_swift: High-performance Go orchestrator for compilation, binary linking, and test execution.