Add unit tests for the Wave corpus checker

## Problem

`tools/check_wave_corpus.py` is a required CI gate for examples and standard-library sources, but it has no dedicated unit tests.

## Scope

Add isolated Python tests using temporary directories and a small fake compiler process. Cover:

- Explicit `--wavec` selection.
- The `WAVEC` environment variable.
- Default local build candidates.
- Deterministic example/std source discovery.
- A successful compiler result.
- A nonzero compiler result with stdout/stderr details.
- Per-file timeout handling.
- Installation of the repository stdlib into an isolated temporary `HOME`.

Do not require LLVM or a real Wave compiler in these unit tests.

## Completion criteria

- [ ] Tests are deterministic on Linux, macOS, and Windows runners.
- [ ] No test reads or writes the developer's real home directory.
- [ ] CI runs the new test module with the existing Python tooling tests.
- [ ] Production corpus behavior is unchanged.

Start with tools/check_wave_corpus.py and the repository's existing Python tooling-test command. Add deterministic tests using temporary directories and a fake compiler process, covering the listed compiler selection, discovery, result, timeout, and isolated HOME cases. Done means the module runs in CI on Linux, macOS, and Windows without LLVM, a real compiler, or the developer's home directory, while production behavior is unchanged.

Written by the indexing model from the issue text.