feat(dist): bundled-zig link backend for hermetic macOS/Linux/Windows builds

Drive a bundled `zig` as `zig cc` for the AOT link step, supplying lld + CRT
+ libc (musl/glibc/mingw) so `sx build` produces native binaries with no host
toolchain. Default Linux output is static musl (portable-anywhere).

- src/zig_backend.zig: discover zig ($SX_ZIG / bundled-next-to-exe / PATH);
  bundled-vs-PATH provenance gates auto-activation.
- src/target.zig: selectZigLinker + emitZigLinkArgv + zigTargetTriple, dispatched
  before the per-OS branches; macOS/Linux/Windows in scope.
- src/ir/emit_llvm.zig: LLVMNormalizeTargetTriple so vendor-less zig triples
  (e.g. x86_64-windows-gnu) parse to the correct OS/object format (COFF not ELF).
- src/main.zig: --self-contained / --no-self-contained; linux-musl, linux-musl-arm,
  windows-gnu shorthands; de-vendor linux/linux-arm to match the corpus runner.
- examples/1660: Windows Win32 print-42 + exit(0) via kernel32 (ir-only off-Windows).

Auto-activates only for a bundled zig; a PATH-only zig engages under
--self-contained, so native dev/CI builds are never silently rerouted.

Docs: readme Cross-Compilation, design/bundled-zig-link-backend-design.md, current/PLAN-DIST.md.

src/ir/emit_llvm.zig

@@ -255,10 +255,16 @@ pub const LLVMEmitter = struct {
         const llvm_module = c.LLVMModuleCreateWithNameInContext(module_name, ctx);
         const builder = c.LLVMCreateBuilderInContext(ctx);
 
-        // Set target triple
-        const triple_owned = target_config.triple == null;
-        const triple = target_config.triple orelse c.LLVMGetDefaultTargetTriple();
-        defer if (triple_owned) c.LLVMDisposeMessage(@constCast(triple));
+        // Set target triple. Normalize first: zig-scheme, vendor-less triples
+        // (e.g. "x86_64-windows-gnu") would otherwise have "windows" land in
+        // LLVM's vendor slot under its positional parser, leaving OS=unknown
+        // and the object format silently falling back to ELF. Normalization is
+        // LLVM's own reordering — not a hand-maintained translation table.
+        const raw_owned = target_config.triple == null;
+        const raw_triple = target_config.triple orelse c.LLVMGetDefaultTargetTriple();
+        defer if (raw_owned) c.LLVMDisposeMessage(@constCast(raw_triple));
+        const triple = c.LLVMNormalizeTargetTriple(raw_triple);
+        defer c.LLVMDisposeMessage(triple);
 
         c.LLVMSetTarget(llvm_module, triple);