test(backend): lock LLVM type/ABI shapes before A7.1 extraction (A7.1 scaffolding step 1)

Test-first scaffolding for LLVM backend modularization (Phase A7.1) before the
type/ABI helpers move into src/backend/llvm/{types,abi}.zig. Visibility-only
change to the targets — no behavior change. Closes the ARCH-SAFETY "no generic
ABI snapshot" gap.

- 2 new emit_llvm.test.zig tests:
  - abiCoerceParamType across every C-ABI size bucket: <=8 -> i64, 9-16 ->
    [2 x i64], >16 -> ptr, HFA (all-float/all-double, <=4 fields) -> unchanged,
    string -> ptr, slice -> ptr, scalar -> unchanged. Built via a local
    internStruct helper (field slice in the module arena -> no testing-allocator
    leak); asserts against emitter.cached_* + LLVMArrayType2.
  - needsByval: true only for >16-byte non-HFA struct; false for <=16 / HFA /
    string / slice / non-struct.
- 1 new .ir snapshot: 1202-ffi-cc-c-large-aggregate (the canonical callconv(.c)
  >16-byte byval example that directly documents abiCoerceParamType) — pins the
  byval param path end-to-end (5 byval + entry reload + 2 sret from Arena.init).
  Path-free + idempotent (verified across two captures). Suite count unchanged
  (snapshot added to an existing example).
- Widened abiCoerceParamType + needsByval to pub (visibility only;
  abiCoerceParamTypeEx/materializeByvalArg/verifySizes stay private — move with
  callers in sub-step 2). No logic touched.
- Recorded the A7.1 coverage inventory + residual gaps (wasm32 usize->i32 branch,
  fn-ptr large-aggregate 1203/1204) in ARCH-SAFETY.md.

Gate: zig build, zig build test, bash tests/run_examples.sh -> 361/0 (no churn
beyond the new 1202 .ir).

src/ir/emit_llvm.zig

@@ -4718,7 +4718,7 @@ pub const LLVMEmitter = struct {
     //   - Small integer struct (≤ 8 bytes, non-HFA) → i64
     //   - HFA (homogeneous float aggregate) → leave as-is (LLVM handles it)
 
-    fn abiCoerceParamType(self: *LLVMEmitter, ir_ty: TypeId, llvm_ty: c.LLVMTypeRef) c.LLVMTypeRef {
+    pub fn abiCoerceParamType(self: *LLVMEmitter, ir_ty: TypeId, llvm_ty: c.LLVMTypeRef) c.LLVMTypeRef {
         return self.abiCoerceParamTypeEx(ir_ty, llvm_ty, true);
     }
 
@@ -4787,7 +4787,7 @@ pub const LLVMEmitter = struct {
         return self.cached_ptr;
     }
 
-    fn needsByval(self: *LLVMEmitter, ir_ty: TypeId, raw_llvm_ty: c.LLVMTypeRef) bool {
+    pub fn needsByval(self: *LLVMEmitter, ir_ty: TypeId, raw_llvm_ty: c.LLVMTypeRef) bool {
         if (self.target_config.isWasm32()) return false;
         if (ir_ty == .string) return false;
         if (!ir_ty.isBuiltin()) {