feat(asm): Phase 2 — -> @place write-through outputs

An asm result can be STORED through a place (a local / struct field) instead of
returned; the place output does not join the result tuple.

- parser.zig: `-> @place` parses `@place` as an ordinary address-of expression
  → an out_place operand (the in-function form; reuses the existing `@` prefix).
- inst.zig: AsmOperand gains out_ty (the output slot's value type) so emit can
  build the combined return struct without re-deriving from Inst.ty.
- lower/expr.zig: out_place operand = the lowered @place address, out_ty = the
  pointee. Read-write (`+`) and indirect-memory (`*`) constraints rejected loudly
  (not yet implemented) rather than miscompiled.
- ops.zig emitInlineAsm: the LLVM return type is built from ALL outputs
  (out_value + out_place); after the call, out_place slots are stored through
  their address and out_value slots rebuild the sx result. Fast path when there
  are no place outputs (the struct return IS the result — pure-value asm IR
  unchanged).

Verified: write-to-local (42), struct field, mixed value+place (v=10 b=20), `+`
rejected. Locked with 1649-platform-asm-place-output (mixed, runs on aarch64).

zig build test green (657 corpus, 446 unit).

examples/1649-platform-asm-place-output.sx

@@ -0,0 +1,19 @@
+// ASM stream Phase 2 — `-> @place` write-through output. An asm result can be
+// STORED through a place (a local / struct field) instead of returned: the
+// place output does NOT join the result tuple. Here one value output is
+// returned (into `main_val`) while a second is written through `@other`. The
+// two are combined to 42. Read-write (`+`) and indirect (`*`) place outputs are
+// not yet implemented (rejected at lowering). aarch64-pinned; ir-only elsewhere.
+compute :: () -> i64 {
+    other : i64 = 0;
+    main_val := asm volatile {
+        #string ASM
+        mov %[m], #5
+        mov %[o], #37
+ASM,
+        [m] "=r" -> i64,        // value output → returned
+        [o] "=r" -> @other,     // place output → stored through @other
+    };
+    return main_val + other;    // 5 + 37 = 42
+}
+main :: () -> i64 { return compute(); }

examples/expected/1649-platform-asm-place-output.build

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

examples/expected/1649-platform-asm-place-output.exit

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

examples/expected/1649-platform-asm-place-output.ir

@@ -0,0 +1,25 @@
+
+; Function Attrs: nounwind
+define internal i64 @compute() #0 {
+entry:
+  %alloca = alloca i64, align 8
+  store i64 0, ptr %alloca, align 8
+  %asm = call { i64, i64 } asm sideeffect "        mov ${0}, #5\0A        mov ${1}, #37\0A", "=r,=r"()
+  %asm.out = extractvalue { i64, i64 } %asm, 0
+  %asm.out1 = extractvalue { i64, i64 } %asm, 1
+  store i64 %asm.out1, ptr %alloca, align 8
+  %allocaN = alloca i64, align 8
+  store i64 %asm.out, ptr %allocaN, align 8
+  %load = load i64, ptr %allocaN, align 8
+  %loadN = load i64, ptr %alloca, align 8
+  %add = add i64 %load, %loadN
+  ret i64 %add
+}
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @compute()
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1649-platform-asm-place-output.stderr

@@ -0,0 +1 @@
+

examples/expected/1649-platform-asm-place-output.stdout

@@ -0,0 +1 @@
+