Explicit modules for native Apple builds

Once can now use the Swift compiler's dependency scan to build imported Swift and Clang modules as separate cacheable actions. Enable explicit_modules on an Apple target or a native Xcode or Swift package workspace. Native Xcode projects also honor SWIFT_ENABLE_EXPLICIT_MODULES = YES.

Shared module work reuses the action cache, and independent declared actions can run concurrently within the build's memory budget. Module identity includes compiler and development-kit contents. Existing builds keep their implicit-module behavior unless explicit modules are selected.

An optional dependency_check = "error" rejects source imports of workspace modules that are not declared dependencies. Errors include the target and a suggested repair. Xcode's inferred import edges do not satisfy this check.

The module authoring contract now includes deferred action planning and host directory digests. These are ecosystem-neutral primitives that other target kinds can use. This release also fixes signing for standalone Apple executables. Parent-relative dependency references now resolve correctly in nested Apple packages.

See Apple builds for configuration and current limits.