test: migrate cross-arch tests to declarative test.toml; add WindowsViaWine

Collapses three outer-driver tests into declarative top-level fixtures:

  - CrossArchAarch64: outer + inner pair becomes main.cpp + test.toml. The
    'is this really an ARM aarch64 ELF?' artifact-introspection check is
    dropped — qemu-aarch64 refuses to run wrong-arch ELFs anyway, so a
    silent host-arch fallback would still fail the run.
  - Wasi: outer + inner pair becomes main.cpp + test.toml. The WASM
    magic-byte check is dropped on the same logic (wasmtime refuses
    non-WASM input).
  - Defines: simple defines-propagation smoke test becomes test.toml with
    [defines] CRAFTER_TEST_FOO = "42".

Adds WindowsViaWine to replace the (forthcoming-deletion) WindowsViaSsh:
declarative target=x86_64-w64-mingw32 + requires=[tool:wine,
tool:x86_64-w64-mingw32-g++]. Exercises the new Wine runner and the
ForTarget derivation end-to-end.

Diamond and CrossProjectModule had speculative --target= reading that
made them attempt cross-compilation under the multi-target sweep. They
have no sysroot/toolchain plumbing, so those cross-builds always fail.
Hardcoded them to HostTarget(); cross-arch tests live in their own
test.toml from now on.

Refs issue #8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/WindowsViaWine/main.cpp

@@ -0,0 +1,6 @@
+import std;
+
+int main() {
+    std::println("hi from win{}", sizeof(void*) * 8);
+    return 0;
+}