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).

examples/expected/1648-platform-asm-global.build

@@ -0,0 +1 @@
+{ "aot": true, "target": "macos" }

examples/expected/1648-platform-asm-global.exit

@@ -0,0 +1 @@
+42

examples/expected/1648-platform-asm-global.ir

@@ -0,0 +1,16 @@
+
+module asm ".global _my_add"
+module asm "_my_add:"
+module asm "    add x0, x0, x1"
+module asm "    ret"
+
+; Function Attrs: nounwind
+declare i64 @my_add(i64, i64) #0
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @my_add(i64 40, i64 2)
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1648-platform-asm-global.stderr

@@ -0,0 +1 @@
+

examples/expected/1648-platform-asm-global.stdout

@@ -0,0 +1 @@
+