ffi 1.6: objc_msg_send IR opcode + per-call-site LLVM fn type

102/102 regression tests pass; chess Android + iOS-sim still build
clean. `ffi-objc-call-04-primitive-returns` flips from xfail to
passing with both nil-recv and real-recv flavors of *void / s64
returns exercised.

Key change: a new `objc_msg_send` IR opcode bundles (recv, sel,
extra args) and carries the return type via the `Inst.ty` field.
emit_llvm.zig builds a per-call-site LLVM function type from the
argument Refs' IR types (recv/sel as ptr; extra args through
abiCoerceParamType) and dispatches with LLVMBuildCall2. One
declared `@objc_msgSend` symbol is reused across every return
type — opaque pointers make the function value type-erased, so
each call site picks its own ABI.

  before:  one (recv, sel) -> ptr LLVM declaration, hard-coded
           per call site; only void return wired in 1.3.
  after:   same declaration, each call site provides a fresh
           LLVMBuildCall2 fn-type → s64 / *void / bool / f64
           returns all dispatch correctly without separate FuncIds.

Selector init mechanism: stayed with the @llvm.global_ctors
constructor. Investigated clang's
`__DATA,__objc_selrefs` + `externally_initialized` shape — works
for fully-linked binaries (dyld substitutes the SEL at load
time) but **LLVM ORC JIT** (the engine behind `sx run`) doesn't
process Mach-O Obj-C metadata sections, so the slot keeps its
initial value (the method-name string pointer) and dispatch
crashes with "<null selector>". The portable choice: keep the
constructor AND inject a direct call to it at `main`'s entry —
idempotent under dyld (sel_registerName returns the same SEL on
re-registration), required for ORC JIT.

Files touched:
  src/ir/inst.zig    | new ObjcMsgSend struct + opcode
  src/ir/lower.zig   | drop the void-only restriction; emit the
                       new opcode; remove the orphaned
                       getObjcMsgSendFid path (objc_msgSend
                       declaration moved to emit_llvm)
  src/ir/emit_llvm.zig | objc_msg_send arm (per-call-site
                       LLVMBuildCall2); lazy `@objc_msgSend`
                       declaration via getObjcMsgSendValue;
                       emitObjcSelectorInit refactored to inject
                       the ctor call at main's entry
  src/ir/{print,interp}.zig | switch arms for the new opcode

`ffi-objc-call-03-selector-sharing.ir` snapshot updates to
reflect the new shape (the `call ... @objc_msgSend` call sites
no longer mention a typed wrapper).

tests/expected/ffi-objc-call-03-selector-sharing.ir

@@ -218,6 +218,7 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 define i32 @main() #0 {
 entry:
+  call void @__sx_objc_selector_init()
   %alloca = alloca { i64 }, align 8
   store { i64 } zeroinitializer, ptr %alloca, align 8
   %si = insertvalue { ptr, ptr, ptr } undef, ptr %alloca, 0
@@ -227,13 +228,13 @@ entry:
   %siN = insertvalue { { ptr, ptr, ptr }, ptr } %siN, ptr null, 1
   store { { ptr, ptr, ptr }, ptr } %siN, ptr @context, align 8
   %load = load ptr, ptr @OBJC_SELECTOR_REFERENCES_init, align 8
-  %call = call ptr @objc_msgSend(ptr null, ptr %load)
+  call void @objc_msgSend(ptr null, ptr %load)
   %loadN = load ptr, ptr @OBJC_SELECTOR_REFERENCES_init, align 8
-  %callN = call ptr @objc_msgSend(ptr null, ptr %loadN)
+  call void @objc_msgSend(ptr null, ptr %loadN)
   %loadN = load ptr, ptr @OBJC_SELECTOR_REFERENCES_init, align 8
-  %callN = call ptr @objc_msgSend(ptr null, ptr %loadN)
+  call void @objc_msgSend(ptr null, ptr %loadN)
   %loadN = load ptr, ptr @OBJC_SELECTOR_REFERENCES_release, align 8
-  %callN = call ptr @objc_msgSend(ptr null, ptr %loadN)
+  call void @objc_msgSend(ptr null, ptr %loadN)
   %allocaN = alloca { ptr, i64 }, align 8
   %gep = getelementptr inbounds { ptr, i64 }, ptr %allocaN, i32 0, i32 0
   store ptr null, ptr %gep, align 8
@@ -245,8 +246,8 @@ entry:
   store { ptr, i64 } { ptr @str.1, i64 0 }, ptr %allocaN, align 8
   %loadN = load { ptr, i64 }, ptr %allocaN, align 8
   %loadN = load { ptr, i64 }, ptr %allocaN, align 8
-  %callN = call { ptr, i64 } @substr({ ptr, i64 } %loadN, i64 0, i64 3)
-  %callN = call { ptr, i64 } @concat({ ptr, i64 } %loadN, { ptr, i64 } %callN)
+  %call = call { ptr, i64 } @substr({ ptr, i64 } %loadN, i64 0, i64 3)
+  %callN = call { ptr, i64 } @concat({ ptr, i64 } %loadN, { ptr, i64 } %call)
   store { ptr, i64 } %callN, ptr %allocaN, align 8
   %loadN = load { ptr, i64 }, ptr %allocaN, align 8
   %str.ptr = extractvalue { ptr, i64 } %loadN, 0
@@ -269,14 +270,12 @@ entry:
   ret void
 }
 
-; Function Attrs: nounwind
-declare ptr @sel_registerName(ptr) #0
-
-; Function Attrs: nounwind
-declare ptr @objc_msgSend(ptr, ptr) #0
+declare ptr @objc_msgSend(ptr, ptr)
 
 declare i64 @write(i32, ptr, i64)
 
+declare ptr @sel_registerName(ptr)
+
 define internal void @__sx_objc_selector_init() {
 entry:
   %sel = call ptr @sel_registerName(ptr @OBJC_METH_VAR_NAME_)

tests/expected/ffi-objc-call-04-primitive-returns.exit

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

tests/expected/ffi-objc-call-04-primitive-returns.txt

@@ -1 +1,4 @@
-/Users/agra/projects/sx/examples/ffi-objc-call-04-primitive-returns.sx: error: #objc_call: only `void` return + (recv, selector) is lowered today; non-void / arg-bearing arities land in later phase-1 steps
+nil class    = true
+nil hash     = 0
+meta non-null = true
+hash non-zero = true