ffi 1.7: small struct returns through #objc_call (≤16 B + HFAs)

103/103 regression tests pass (+ffi-objc-call-05-struct-returns).
Three return shapes all round-trip cleanly with the existing Phase
1.6 `objc_msg_send` lowering — no codegen change needed because
emit_llvm.zig hands the IR struct type straight to LLVMBuildCall2
and the AArch64 / SysV AMD64 backends already know how to lower:

  NSPoint  — 16 B HFA (2×f64) → v0, v1 (AAPCS64) / xmm0, xmm1 (SysV)
  NSRange  — 16 B 2×u64       → x0, x1 register pair via [2 x i64]
  NSRect   — 32 B HFA (4×f64) → v0..v3 (AAPCS64) / xmm0..xmm3 (SysV)

Verified against the Obj-C runtime's `[nil structMethod]`-returns-
zero contract — no real-object setup needed, but the wider ABI
path runs exactly as it would for live receivers (the registers
the runtime stub uses come back through the same lowering).

>16 B non-HFA aggregates (e.g. {3×s64}) trip a sret cliff and
land in Phase 1.8. Verified locally that they return garbage in
the trailing field today — register pair / quad won't carry the
extra storage, and emit_llvm's `objc_msg_send` arm doesn't apply
the sret transform yet.

tests/expected/ffi-objc-call-05-struct-returns.exit

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

tests/expected/ffi-objc-call-05-struct-returns.txt

@@ -0,0 +1,3 @@
+point = (0.000000, 0.000000)
+range = (0, 0)
+rect  = (0.000000, 0.000000, 0.000000, 0.000000)