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/lower/decl.zig

@@ -1342,6 +1342,17 @@ pub fn lowerMainAndComptime(self: *Lowering, decls: []const *const Node) void {
                     self.lowerMainAndComptime(ns.decls);
                 }
             },
+            // Top-level global asm (Phase F): capture the verbatim template; it
+            // is appended to the LLVM module at emit time (source order). The
+            // template must be a comptime-known string (parser guarantees a
+            // string node here).
+            .asm_global => |ag| {
+                if (ag.template.data == .string_literal) {
+                    self.module.global_asm.append(self.alloc, ag.template.data.string_literal.raw) catch unreachable;
+                } else if (self.diagnostics) |diags| {
+                    diags.addFmt(.err, decl.span, "global asm template must be a compile-time-known string", .{});
+                }
+            },
             else => {},
         }
     }