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>

implementations/Crafter.Build-Platform.cpp

@@ -778,6 +778,12 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
         std::string archFlags = isWasm
             ? std::string(" -fno-exceptions -msimd128 -fno-c++-static-destructors -mllvm -wasm-enable-sjlj -D_WASI_EMULATED_SIGNAL")
             : std::format(" -march={} -mtune={}", config.march, config.mtune);
+        // Match the variant codegen flags the consuming TUs are built with
+        // (e.g. -mrelaxed-simd) so the std BMI is feature-compatible. wasm only;
+        // wasmVariantFlags is empty on every other path.
+        if (isWasm) {
+            for (const std::string& f : config.wasmVariantFlags) archFlags += " " + f;
+        }
         if(!fs::exists(stdPcm) || fs::last_write_time(stdPcm) < fs::last_write_time(stdCppm)) {
             return RunCommand(std::format("clang++ --target={} -std=c++26 -stdlib=libc++{}{} -O3 -Wno-reserved-identifier -Wno-reserved-module-identifier --precompile {} -o {}", config.target, sysrootFlag, archFlags, stdCppm, stdPcm.string()));
         } else {