Expand LLVM native output targets (#280)

* Expand LLVM native output targets

- Add owned helper and runtime-pack contracts for macOS x64, Linux glibc/musl, Windows x64, and WASI.
- Generalize LLVM target selection, package validation, linker plans, and release/CI matrices.
- Document linker boundaries and add focused native-output and runtime-pack coverage.

* Pin Windows LLVM helper CI to VS 2022

The official LLVM archive needs the Visual Studio 2022 CMake generator, which is not guaranteed by windows-latest.

* Align bootstrap cache with LLVM runtime-pack builds

Use the same target-specific helper and linker identity before and after compiler loading so routed executable cache hits remain lightweight.

* Install Windows LLVM CI tools with Chocolatey

windows-2022 includes Visual Studio but not winget; use its available Chocolatey bootstrap for CMake and Ninja.

* Prevent Zig version probes from leaving runtime-pack objects

Run runtime-pack compiler version probes in a private temporary directory and remove the accidental tracked WASI a.o file.

* Use 7-Zip for Windows LLVM setup

Extract the official LLVM development archive with two-stage 7-Zip instead of Windows tar.exe, which timed out while materializing the toolchain tree.

* Define ssize_t for the MSVC runtime pack

Clang's MSVC target does not expose POSIX ssize_t through sys/types.h; define the pointer-sized runtime type without affecting MinGW.

* Build the Windows runtime pack with Zig

Use the MinGW-compatible Windows sysroot required by the runtime while retaining COFF helper output and installing Zig in the Windows native CI lane.

* Give macOS LLVM differential shard time to finish

Shard 1 passed its setup and focused contracts but was cancelled during its cold LLVM differential slice at the 20-minute job limit.