Add a Fuchsia SDK and runtime target contract

## Goal

Implement a backend-neutral Fuchsia target contract for the amd64 and arm64 roadmap suites introduced by #457, with riscv64 retained only as a future reservation until the platform publishes a usable contract.

## Scope

- Define exact target triples, Zircon/Fuchsia SDK discovery, object/link modes, and runtime entry contract.
- Identify supported libc/system interfaces and unavailable POSIX assumptions in `std`.
- Add SDK-built C ABI fixtures and object validation.
- Support compile-only artifacts before claiming emulator/runtime support.
- Keep the target representation consumable by Whale.

## Completion criteria

- The first supported Fuchsia architecture emits and links against an explicit SDK.
- Missing SDK configuration produces a structured diagnostic.
- Standard-library capabilities are documented and tested.
- CI activation matches the verified support level.

Related: #93, #368, #457

Begin with the roadmap suites introduced by #457 and the target representation consumed by Whale. Verify the SDK-built C ABI fixtures, object validation, standard-library documentation and tests, and CI activation against the completion criteria, including compile-only support before runtime claims.

Written by the indexing model from the issue text.