feat(asm): Phase F — global (module-scope) asm

A top-level `asm { "tmpl", };` block (template only) lowers to LLVM `module asm`;
a lib-less `extern` declaration calls into the symbols it defines (the import
direction reuses the existing C-FFI extern path — no new surface).

- ast.zig: asm_global node (AsmGlobal { template }).
- parser.zig: parseAsmGlobal, dispatched from parseTopLevel on kw_asm — rejects
  `volatile` and any operands/clobbers (template only). The in-function asm
  expression form stays in parsePrimary.
- module.zig: Module.global_asm list; lower/decl.zig captures each template in
  lowerMainAndComptime (the real top-level pass — lowerDecls is dead for
  top-level); emit_llvm.zig emit() appends each via LLVMAppendModuleInlineAsm in
  source order.
- the new node forced asm_global arms in sema.zig (analyzeNode +
  findNodeAtOffset) and semantic_diagnostics.zig (checkBindingNames).

Verified end-to-end: an aarch64 `_my_add` global routine, called via `extern`,
returns 42 — AOT only (the ORC JIT doesn't link module-asm symbols; global-asm
symbols live in the final linked binary). Locked with 1648-platform-asm-global
({ "aot": true, "target": "macos" } → AOT build+run on aarch64, ir-only else).

zig build test green (656 corpus, 446 unit).

src/ir/emit_llvm.zig

@@ -347,6 +347,14 @@ pub const LLVMEmitter = struct {
         // Must precede any DISubprogram (created per function below).
         self.debugInfo().initDebugInfo();
 
+        // Top-level global asm (ASM stream Phase F): append each block verbatim
+        // to the module. Multiple blocks concatenate in source order; LLVM emits
+        // them as module-level `module asm`. Symbols they define are reached via
+        // lib-less `extern` declarations.
+        for (self.ir_mod.global_asm.items) |asm_text| {
+            c.LLVMAppendModuleInlineAsm(self.llvm_module, asm_text.ptr, asm_text.len);
+        }
+
         // Pass 0: Declare and initialize globals
         self.emitGlobals();