ffi M5.A.next.5.2.A: generic Into(Block) impl — xfail lock-in

`examples/177-generic-into-block.sx` exercises a closure shape
(`Closure(s64, s64) -> void`) that stdlib's hand-rolled
`Into(Block)` impls don't cover. Today: the focused diagnostic
"no `Into(Block) for cl_s64_s64__void` impl — add a
per-signature `__block_invoke_<sig>` trampoline + Into impl
alongside the existing ones in modules/std/objc_block.sx, or
declare it in your own code" fires at the `xx cl : Block` site.

The next commit adds the generic
`impl Into(Block) for Closure(..$args) -> $R` to
`library/modules/std/objc_block.sx` (wiring `#insert
build_block_convert($args, $R)` from step 5.1.B) plus the
lowering plumbing needed to make pack + single-type `$` refs
work inside the impl's monomorphisation. The test then flips
green — the per-shape trampoline emitted by build_block_convert
ferries (10, 20) through to the sx closure and the side-effect
stores land in g_a / g_b.

examples/177-generic-into-block.sx

@@ -0,0 +1,37 @@
+// FFI plan step 5.2 — generic `Into(Block) for Closure(..$args) ->
+// $R` impl. One impl in stdlib covers every closure shape; the
+// compiler monomorphises the impl body per call shape and emits a
+// dedicated `__invoke` `callconv(.c)` trampoline + Block literal
+// (via `#insert build_block_convert($args, $R);`).
+//
+// This test exercises a closure shape (`Closure(s64, s64) -> void`)
+// that has NO hand-rolled `Into(Block)` impl in
+// `library/modules/std/objc_block.sx`. Before step 5.2 lands,
+// `xx cl : Block` errors out with the "no Into(Block) for
+// cl_s64_s64__void" focused diagnostic. After the generic impl
+// lands, the same call resolves through the pack-shaped impl and
+// the per-shape trampoline ferries control back to the sx closure.
+//
+// The block is invoked directly through `b.invoke` (a typed
+// `callconv(.c)` fn-pointer) — the same shape the Apple Block
+// runtime calls when a UIKit/Foundation API hands the block back
+// to its registered invoke.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+g_a: s64 = 0;
+g_b: s64 = 0;
+
+main :: () -> s32 {
+    cl := (a: s64, b: s64) => { g_a = a; g_b = b; };
+    blk : Block = xx cl;
+
+    invoke_fn : (*Block, s64, s64) -> void callconv(.c) = xx blk.invoke;
+    invoke_fn(@blk, 10, 20);
+
+    if g_a != 10 { print("FAIL: g_a={}\n", g_a); return 1; }
+    if g_b != 20 { print("FAIL: g_b={}\n", g_b); return 1; }
+    print("generic-into-block ok: a={} b={}\n", g_a, g_b);
+    0;
+}

tests/expected/177-generic-into-block.exit

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

tests/expected/177-generic-into-block.txt

@@ -0,0 +1 @@
+generic-into-block ok: a=10 b=20