ffi block-string-arg ABI mismatch — expected-failing lock-in

Generic `Into(Block) for Closure(string) -> void` (step 5.2) emits
a trampoline whose `callconv(.c)` param type collapses through
`abiCoerceParamType`'s `string → ptr` heuristic — the libc
"char *" convention. The caller side (typed fn-pointer cast +
indirect call through `b.invoke`) keeps the full `{ptr, i64}`
slice. Result on AArch64: caller passes 16 bytes in x0+x1,
trampoline reads 8 bytes from x0 only, the slice len is lost or
mis-tracked, and the trampoline's `memcpy` from the half-formed
string segfaults.

`examples/188-block-string-arg.sx` pins the post-fix behaviour
("got: <hello>"). Today's run segfaults inside the trampoline's
first read. The next commit splits `abiCoerceParamType` into a
foreign-only path (extern decls keep the libc collapse) and a
preserve-slice path (sx-internal `callconv(.c)`).

examples/188-block-string-arg.sx

@@ -0,0 +1,28 @@
+// Generic `Into(Block)` impl with a `string`-typed arg in the
+// closure signature. The block trampoline declares the param with
+// callconv(.c); without the abi-collapse fix, sx `string` got
+// silently collapsed to `ptr` (the libc `char *` heuristic) and
+// the caller's 16-byte `{ptr, len}` value mismatched the
+// trampoline's 8-byte `ptr` slot. Result: segfault inside the
+// trampoline's first read.
+//
+// The fix lives in `abiCoerceParamTypeEx`: the `string`/`slice` →
+// `ptr` collapse only applies to `is_extern` foreign decls (libc
+// interop). sx-internal `callconv(.c)` keeps the full slice
+// shape, which lands as `[2 x i64]` at the LLVM signature site
+// and matches the caller's two-register pass on AArch64.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+g_s: string = "";
+
+main :: () -> s32 {
+    cl := (s: string) => { g_s = s; };
+    b : Block = xx cl;
+    invoke_fn : (*Block, string) -> void callconv(.c) = xx b.invoke;
+    invoke_fn(@b, "hello");
+    if g_s.len == 0 { print("FAIL: empty\n"); return 1; }
+    print("got: <{}>\n", g_s);
+    0;
+}