Expose Whale target and emit-kind capability queries #886

Motivation

wavec --whale print supported-targets is intentionally rejected by src/cli.rs even though the adapter has a definite one-target, one-output capability subset. Existing tooling cannot discover it through the established print interface.

Scope

Implement supported-targets/target-list and supported-emit-kinds using the existing human/JSON schemas. Advertise only Linux amd64 and IR; check is a validation mode, and native outputs are unavailable. Keep all other unsupported queries explicit and LLVM default behavior unchanged.

Difficulty: easy.

Completion criteria

• Return accurate human and JSON capabilities for Whale.
• Verify aliases and invalid query handling.
• Do not infer Whale capabilities from enabled LLVM targets.