feat: feature-detected browser-wasm variants (relaxed-SIMD) + variant-aware runtime

The browser wasm pipeline hardcoded -msimd128 for every wasm32 target and
baked a single wasm URL into index.html, so newer codegen features that
aren't yet baseline across engines (relaxed SIMD today; threads, future SIMD
revisions later) couldn't be adopted without dropping the browsers that lack
them.

Add a general, feature-parameterized mechanism owned entirely by
Crafter.Build:

- Configuration::wasmVariants declares N codegen variants (label, extra -m
  flags, runtime probes). Build() compiles the baseline plus one
  outputName.<label>.wasm per variant, recompiling the whole graph (incl.
  dep libs + std PCM) with the variant's flags — relaxed-SIMD is per-TU
  codegen, not a link switch. wasmVariantFlags folds into VariantId so each
  variant's objects/PCMs land in their own build+bin dir.
- EnableWasiBrowserRuntime emits a variants.json manifest (label -> url +
  probes), preferred-first with the baseline as the universal fallback.
- The shipped runtime.js runs inlined wasm-feature-detect probes
  (relaxed-simd, simd, tail-call, bulk-memory, exception-handling, threads),
  picks the first variant whose probes all pass, and falls back to the single
  baked CRAFTER_WASM_URL when no manifest is present (backward compatible).
- EnableWasiRelaxedSimdVariant registers the relaxed-SIMD variant — the
  motivating case (Chrome 114+/Firefox 120+ enable it by default; Safari
  still flag-gates it as of mid-2026).

Verified end to end: a wasm32-wasip1 build emits both wasi-hello.wasm and
wasi-hello.relaxed-simd.wasm + variants.json; Firefox selects the
relaxed-simd variant and runs it.

Resolves #24

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

project.cpp

@@ -106,6 +106,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("TestRunnerSpec").Dependencies({ crafterBuildLib.get() });
         cfg.AddTest("VariantId").Dependencies({ crafterBuildLib.get() });
         cfg.AddTest("WasiBrowserRuntime").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("WasmVariants").Dependencies({ crafterBuildLib.get() });
         cfg.AddTest("RunSingleTestExit").Dependencies({ crafterBuildLib.get() });
         // LoadProject dlopens the synthesized project.so, which references
         // Crafter:: symbols (HostTarget, Configuration ctors) that have to be