Document the exact Wave target support and validation matrix

## Why

The compiler registry currently supports exact triples across Linux, macOS, Windows GNU, and freestanding ELF, but the README only shows a few commands. Contributors need one authoritative table that distinguishes accepted targets from planned targets and states what CI actually executes.

## Scope

- Add a concise target matrix to the README or a focused target-support document linked from it.
- Derive exact triples from `llvm/src/codegen/target.rs`.
- For each target, state architecture, object format, hosted/freestanding status, default ABI where relevant, linker/runtime requirement, and current CI validation level.
- Link the `wavec print target-list`, `target-spec`, and `sysroot` commands.
- Clearly mark musl, MSVC, FreeBSD, WebAssembly, and other planned targets as unsupported until their implementation issues land.

## Completion criteria

- [ ] Every registered triple appears exactly once.
- [ ] The table does not claim runtime execution where CI only compiles.
- [ ] Hosted cross-linking and freestanding behavior are explained.
- [ ] The information matches current target specs and workflows.

Start with llvm/src/codegen/target.rs to enumerate registered triples, then inspect the README and CI workflows for the commands and validation they actually run. Add the target matrix and links, distinguishing compile-only checks from runtime execution and marking planned targets unsupported. Verify every registered triple appears once and that the table matches the target specs and workflows.

Written by the indexing model from the issue text.