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/StandardArgs/main.cpp

@@ -0,0 +1,75 @@
+import std;
+import Crafter.Build;
+using namespace Crafter;
+
+namespace {
+    int failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+}
+
+// Pure-data tests for ApplyStandardArgs and ArgQuery — no build/run, just
+// confirms the documented arg parsing produces the documented mutations.
+int main() {
+    // --debug + --target= + --march= + --mtune=
+    {
+        Configuration cfg;
+        cfg.target = "ignored";
+        std::array<std::string_view, 4> raw = {
+            "--debug", "--target=aarch64-linux-gnu", "--march=armv8-a", "--mtune=cortex-a72",
+        };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.debug, "--debug should set cfg.debug");
+        Check(cfg.target == "aarch64-linux-gnu", "--target= should overwrite cfg.target");
+        Check(cfg.march == "armv8-a", "--march= should overwrite cfg.march");
+        Check(cfg.mtune == "cortex-a72", "--mtune= should overwrite cfg.mtune");
+    }
+
+    // --lib promotes Executable -> LibraryStatic; --shared then promotes to Dynamic.
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        std::array<std::string_view, 1> raw = { "--lib" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::LibraryStatic, "--lib promotes Exe -> Static");
+    }
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        // Order shouldn't matter — promotions chain in priority order.
+        std::array<std::string_view, 2> raw = { "--shared", "--lib" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::LibraryDynamic, "--lib + --shared lands on Dynamic regardless of order");
+    }
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        std::array<std::string_view, 1> raw = { "--shared" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::Executable, "--shared alone on Executable is a no-op");
+    }
+
+    // ArgQuery is returned over the same args span and exposes Has/Get.
+    {
+        Configuration cfg;
+        std::array<std::string_view, 3> raw = { "--timing", "--prefix=/opt/x", "extra" };
+        ArgQuery q = ApplyStandardArgs(cfg, raw);
+        Check(q.Has("--timing"), "ArgQuery::Has true for present flag");
+        Check(!q.Has("--missing"), "ArgQuery::Has false for absent flag");
+        auto prefix = q.Get("--prefix=");
+        Check(prefix.has_value(), "ArgQuery::Get returns a value for known prefix");
+        Check(prefix.value_or("") == "/opt/x", "ArgQuery::Get returns the substring after =");
+        Check(!q.Get("--other=").has_value(), "ArgQuery::Get returns nullopt for absent prefix");
+    }
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}