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/ast.zig

@@ -96,6 +96,7 @@ pub const Node = struct {
         runtime_class_decl: RuntimeClassDecl,
         jni_env_block: JniEnvBlock,
         asm_expr: AsmExpr,
+        asm_global: AsmGlobal,
 
         pub fn declName(self: Data) ?[]const u8 {
             return switch (self) {
@@ -259,6 +260,15 @@ pub const AsmOperand = struct {
     };
 };
 
+/// Top-level (module-scope) global assembly: `asm { "tmpl", };` (ASM stream
+/// design §II.2 Deviation 6). Template only — no operands, no `volatile`, no
+/// `clobbers`, no `%` substitution. Lowers to `LLVMAppendModuleInlineAsm`;
+/// multiple blocks concatenate in source order. Symbols it defines are reached
+/// with a lib-less `extern` declaration.
+pub const AsmGlobal = struct {
+    template: *Node, // string-literal / `#string` heredoc node
+};
+
 pub const Identifier = struct {
     name: []const u8,
     /// True when written as a backtick raw identifier (`` `i2 ``). Carried so a