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/semantic_diagnostics.zig

@@ -316,6 +316,7 @@ pub const UnknownTypeChecker = struct {
                 self.checkBindingNames(ae.template);
                 for (ae.operands) |op| self.checkBindingNames(op.payload);
             },
+            .asm_global => |ag| self.checkBindingNames(ag.template),
             // ── Named type / alias / import declarations: a bare reserved
             // spelling as the declared name is rejected. These
             //    have no nested binding sites, so only the name is checked. A