ffi M5.A.next.5.1.A: build_block_convert — expected-failing lock-in

Step 5.1.A of the FFI plan (variadic heterogeneous type packs →
generic `Into(Block)` impl). The eventual step-5.2 impl body will
read `#insert build_block_convert($args, $R);` to emit a per-shape
`__invoke` `callconv(.c)` trampoline + Block literal. 5.1.A pins
the builder's expected output verbatim across three void-returning
pack shapes (0, 1, 2 args) plus one non-void shape (`f64 -> s32`)
that exercises the `return typed_fn(...)` branch.

Today: 4× "unresolved 'build_block_convert'" diagnostics — the
builder isn't in stdlib yet. The next commit adds it to
`library/modules/std/objc_block.sx` and the test flips green.

The per-position type names in the emitted source come from
`type_name(args[i])`; the slice itself is `[]Type` flowing through
the new-form variadic + bare-`$args` path that the recent
issues-0048/0049/0050 fixes unblocked.

examples/176-build-block-convert.sx

@@ -0,0 +1,35 @@
+// FFI plan step 5.1 — `build_block_convert(args: []Type, $ret: Type)
+// -> string` emits the per-shape source body for the generic
+// `Into(Block) for Closure(..$args) -> $R` impl that lands in step
+// 5.2. Per-call-shape monomorphisation of the impl body re-runs the
+// builder with concrete types bound, so each closure shape gets its
+// own dedicated `__invoke` trampoline + Block literal.
+//
+// This test exercises the builder directly (no `#insert`, no impl
+// wiring) — three pack shapes through the same `void`-returning
+// wrapper plus one non-void `s32`-returning wrapper to pin the
+// `return typed_fn(...)` branch. The expected output captures the
+// generated source verbatim so any formatting drift surfaces here
+// rather than as a downstream compile error inside the eventual
+// step-5.2 impl.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+preview_void :: (..$args) -> string {
+    return build_block_convert($args, void);
+}
+
+preview_s32 :: (..$args) -> string {
+    return build_block_convert($args, s32);
+}
+
+run_all :: () {
+    print("--- void / 0 args ---\n{}\n", preview_void());
+    print("--- void / bool ---\n{}\n", preview_void(true));
+    print("--- void / s64, string ---\n{}\n", preview_void(42, "hi"));
+    print("--- s32 / f64 ---\n{}\n", preview_s32(3.14));
+}
+#run run_all();
+
+main :: () { print("rt\n"); }

tests/expected/176-build-block-convert.exit

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

tests/expected/176-build-block-convert.txt

@@ -0,0 +1,10 @@
+--- void / 0 args ---
+__invoke :: (block_self: *Block) -> void callconv(.c) { typed_fn : (*void) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- void / bool ---
+__invoke :: (block_self: *Block, arg0: bool) -> void callconv(.c) { typed_fn : (*void, bool) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env, arg0); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- void / s64, string ---
+__invoke :: (block_self: *Block, arg0: s64, arg1: string) -> void callconv(.c) { typed_fn : (*void, s64, string) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env, arg0, arg1); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- s32 / f64 ---
+__invoke :: (block_self: *Block, arg0: f64) -> s32 callconv(.c) { typed_fn : (*void, f64) -> s32 = xx block_self.sx_fn; return typed_fn(block_self.sx_env, arg0); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- build done ---
+rt