Add an AArch64 iOS SDK target contract

## Goal

Implement the AArch64 iOS target plan introduced by #457 using an explicit Apple SDK, deployment target, Mach-O, linker, and ABI contract. This is separate from macOS deployment tracking in #415.

## Scope

- Register a real-device AArch64 iOS target before considering simulator variants.
- Discover the SDK through supported Apple tooling and honor explicit overrides.
- Define deployment version, platform load commands, object/link modes, and code-signing boundaries.
- Validate C ABI and Mach-O metadata against Clang-built fixtures.
- Keep unsupported riscv64 directory coverage planned only.

## Completion criteria

- Object and static-library emission pass on a pinned macOS runner.
- Missing SDK/deployment settings have actionable diagnostics.
- Executable/device support is not claimed without signing and runtime validation.
- Target fixtures are backend-neutral for Whale migration.

Related: #93, #368, #415, #457

Start by reading the AArch64 iOS target plan in #457 and the related issues, then inspect the target and backend areas that handle Apple SDK discovery, deployment settings, Mach-O metadata, linking, and ABI fixtures. Validate object and static-library output on a pinned macOS runner against Clang-built fixtures; done means actionable missing-setting diagnostics and no unsupported executable or device claims without signing and runtime validation.

Written by the indexing model from the issue text.