Establish a 32-bit target baseline with i686 GNU/Linux

## Goal

Wave currently registers only 64-bit architectures. Add `i686-unknown-linux-gnu` as the first 32-bit target so pointer-width assumptions are found and made explicit.

## Scope

- Add an i686 architecture/target spec and x86 LLVM initialization path.
- Make `isz`, `usz`, pointer arithmetic, array indexing, layouts, and constant casts honor a 32-bit data layout.
- Implement the i386 SysV C ABI classifier, including aggregate and stack argument rules.
- Add ELF32 object validation, hosted sysroot/CRT linking, and QEMU/native runtime smoke tests.
- Audit serialization and linker validation code for ELF64-only assumptions.
- Do not add every 32-bit target in this issue.

## Completion criteria

- [ ] A simple freestanding and hosted i686 program emits valid ELF32.
- [ ] `isz`/`usz` are 32-bit for this target and 64-bit targets remain unchanged.
- [ ] Pointer/int conversions reject width loss unless explicit and valid.
- [ ] C ABI edge fixtures pass against a 32-bit C compiler.
- [ ] CI has a bounded compile/link/run contract.

Start by locating the existing 64-bit target specifications, x86 LLVM initialization, ABI classifier, ELF validation, linker paths, and runtime smoke tests. Compare each area with the stated 32-bit requirements, then use freestanding and hosted i686 programs plus C ABI edge fixtures to verify valid ELF32 output, correct pointer widths, conversions, linking, and bounded CI execution.

Written by the indexing model from the issue text.