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).

src/ir/inst.zig

@@ -368,8 +368,15 @@ pub const InlineAsm = struct {
         name: StringId,
         /// Verbatim constraint, e.g. "={rax}", "=r", "+r", "{rdi}", "r".
         constraint: StringId,
-        /// `input` → the value `Ref`; `out_value` → `.none` (the asm yields it).
+        /// `input` → the value `Ref`; `out_value` → `.none` (the asm yields it);
+        /// `out_place` → the place ADDRESS `Ref` (a pointer; the asm result is
+        /// `store`d through it).
         operand: Ref,
+        /// The value type carried by an OUTPUT slot — `out_value`: its result
+        /// type; `out_place`: the pointee type stored through `operand`. `.void`
+        /// for inputs (their type comes from the input `Ref`). Lets emit build
+        /// the combined LLVM return struct without re-deriving from `Inst.ty`.
+        out_ty: TypeId = .void,
 
         pub const Role = enum { out_value, out_place, input };
     };