test: broaden self-test coverage beyond the compile smoke test

The suite had only HelloWorld, which built and exited an empty exe. Add
in-process tests covering each public surface area users actually touch:

- StaticLib / ModuleInterface / DependencyLink — Build() against
  fixtures for libraries, project-local module interfaces, and
  cross-config module deps with link verification (runs the built exe).
- ShaderCompile — drives Shader::Compile directly, validates SPIR-V
  magic + Check() idempotency.
- StandardArgs — covers --debug, --target=, --march=, --mtune=,
  --lib/--shared promotions, and ArgQuery::Has / Get.
- TestRunnerSpec — FromSpec parse rules, ForTarget routing for host,
  wasm32-wasip1, aarch64-linux-gnu (+ sysroot QEMU_LD_PREFIX),
  i686 → qemu-i386 rewrite, mingw → wine on Linux hosts, FromEnv.
- VariantId — confirms type / debug / sysroot / defines / compileFlags /
  target / march all perturb the cache key, plus PcmDir routing.
- WasiBrowserRuntime — calls EnableWasiBrowserRuntime, asserts the
  three cfg.files entries get registered and index.html had its
  template placeholder substituted.
- RunSingleTestExit — drives RunSingleTest against tiny sh scripts and
  pins the documented exit-code mapping (0/77/non-zero) and the
  Cmd-prefix runner path.

Closes #12.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/ModuleInterface/fixture/Greeter.cppm

@@ -0,0 +1,4 @@
+export module Greeter;
+import std;
+
+export std::string Greet() { return "ok-from-module"; }