Make Arch.hostArchitecture() fail fast on unsupported architectures.

[anxkhn]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Arch.hostArchitecture() in Sources/Services/ContainerAPIService/Client/Arch.swift
resolves the host architecture with a compile-time switch:

public static func hostArchitecture() -> Arch {
    #if arch(arm64)
    return .arm64
    #elseif arch(x86_64)
    return .amd64
    #endif
}

The #if/#elseif has no #else arm. On arch(arm64) and arch(x86_64) this
compiles and returns the right value, but on any other architecture the function
declares a non-optional Arch return type yet has no return statement, so the
compiler emits a confusing "missing return in a function expected to return 'Arch'"
diagnostic rather than a clear reason.

This adds an #else arm that fails the build with an explicit message:

#else
#error("unsupported host architecture")
#endif

This makes the conditional exhaustive and turns a vague missing-return error into a
precise one, mirroring how ClientKernel.swift already handles the runtime analogue
of this switch with fatalError("unknown architecture"). It is behavior-preserving
on the two supported architectures: the #else arm is inert there, so there is no
change to generated code or runtime behavior on Apple silicon or Intel.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Built swift build --target ContainerAPIClient before and after the change; both
succeed on arm64, and only Arch.swift recompiles, confirming the #else arm is
inert on a supported architecture. swift format lint --strict reports no issues
and the formatter is a no-op on the file. The existing ArchTests suite passes.

No test was added: hostArchitecture() is resolved entirely at compile time, so the
#else/#error arm is stripped by the compiler and has no runtime code path to
exercise. ArchTests covers the runtime Arch(rawValue:) initializer, which is
unchanged.