Add a Rhea AArch64 and SVE target profile

## Goal

Turn the `linux/rhea` roadmap suite from #457 into an explicit SiPearl Rhea AArch64 CPU profile instead of a separate base architecture.

## Scope

- Define the baseline AArch64/Armv8 contract and required Neoverse V1 features.
- Specify SVE feature selection without hard-coding a vector length into portable Wave semantics.
- Add CPU-tuning and feature-diagnostic fixtures shared by LLVM and Whale.
- Validate Linux ABI compatibility and generated objects independently.
- Integrate with the CPU/profile manifest model rather than duplicating AArch64 cases.

## Completion criteria

- Rhea resolves through the AArch64 base target with an explicit CPU profile.
- Unsupported SVE requests receive structured diagnostics.
- Baseline and SVE-enabled object fixtures pass.
- The Linux profile remains disabled until an executable environment is validated.

Related: #93, #368, #457

Begin with the linux/rhea roadmap suite from #457 and the CPU/profile manifest model, then compare related issues #93 and #368. Inspect the shared LLVM and Whale fixtures and Linux ABI/object validation. Done means Rhea resolves through AArch64, unsupported SVE requests produce structured diagnostics, fixtures pass, and the Linux profile stays disabled until an executable environment is validated.

Written by the indexing model from the issue text.