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/module.zig

@@ -51,6 +51,10 @@ pub const Module = struct {
     /// (trampoline emission, +alloc/-dealloc synthesis) can re-walk
     /// `members` for fields / methods / `#extends` / `#implements`.
     objc_defined_class_cache: std.ArrayList(ObjcDefinedClassEntry),
+    /// Top-level `asm { … }` blocks (ASM stream Phase F), in source order.
+    /// Each is verbatim assembly appended to the LLVM module via
+    /// `LLVMAppendModuleInlineAsm` at emit time; multiple blocks concatenate.
+    global_asm: std.ArrayList([]const u8),
     alloc: Allocator,
     /// Owns the per-instruction operand slices the Builder dupes (aggregate
     /// fields, call args, branch args, switch cases, block params). These live
@@ -100,6 +104,7 @@ pub const Module = struct {
             .objc_selector_cache = std.ArrayList(ObjcSelectorEntry).empty,
             .objc_class_cache = std.ArrayList(ObjcClassEntry).empty,
             .objc_defined_class_cache = std.ArrayList(ObjcDefinedClassEntry).empty,
+            .global_asm = std.ArrayList([]const u8).empty,
             .alloc = alloc,
             .slice_arena = std.heap.ArenaAllocator.init(alloc),
         };
@@ -115,6 +120,7 @@ pub const Module = struct {
         self.objc_selector_cache.deinit(self.alloc);
         self.objc_class_cache.deinit(self.alloc);
         self.objc_defined_class_cache.deinit(self.alloc);
+        self.global_asm.deinit(self.alloc);
         self.types.deinit();
         self.slice_arena.deinit();
     }