ffi 0.9: foreign-result chains — handle threaded through struct + List

96/96 regression tests pass (+ffi-09-foreign-result-chain).

Opaque C-handle pattern that mirrors how real sx code threads
MTLBuffer*, AAssetManager*, file pointers, etc. through composite
sx values. C side has a trivial heap-int handle (`ffi_chain_make`
returning `void*`, `ffi_chain_bump` / `_peek` / `_dispose`). The sx
side exercises:

  1. Chained calls   — make -> bump -> bump -> peek; one handle
                       threaded through four FFI sites in sequence.
  2. Struct field    — `Counter { handle: *void; label: string; }`
                       hosts the handle; methods/accesses go through
                       `.handle` to feed back into C.
  3. List(*void)     — push N handles, iterate, peek each, iterate
                       again to bump each, iterate again to read
                       back. Catches any aliasing / lifetime breakage
                       when handles round-trip through the slice
                       backing of List.

examples/ffi-09-foreign-result-chain.c

@@ -0,0 +1,22 @@
+#include "ffi-09-foreign-result-chain.h"
+#include <stdlib.h>
+
+void *ffi_chain_make(int seed) {
+    int *p = (int *)malloc(sizeof(int));
+    if (p) *p = seed;
+    return p;
+}
+
+int ffi_chain_bump(void *h, int delta) {
+    int *p = (int *)h;
+    *p += delta;
+    return *p;
+}
+
+int ffi_chain_peek(void *h) {
+    return *(int *)h;
+}
+
+void ffi_chain_dispose(void *h) {
+    free(h);
+}

examples/ffi-09-foreign-result-chain.h

@@ -0,0 +1,10 @@
+// Trivial opaque-handle pattern — `make` produces a heap-allocated
+// counter, `bump` returns the new value, `peek` reads without
+// mutating, `dispose` frees. Mirrors the shape of real C handles
+// (MTLBuffer*, AAssetManager*, file pointers, etc.) without pulling
+// in any platform deps.
+
+void *ffi_chain_make    (int seed);
+int   ffi_chain_bump    (void *h, int delta);
+int   ffi_chain_peek    (void *h);
+void  ffi_chain_dispose (void *h);

examples/ffi-09-foreign-result-chain.sx

@@ -0,0 +1,79 @@
+// Phase 0 baseline (PLAN-FFI.md step 0.9): FFI result chains. The
+// shapes that real sx code uses for opaque C handles (MTLBuffer*,
+// AAssetManager*, file pointers, ...) — passing a C-returned
+// pointer into another C call, stashing it in a struct field,
+// pushing into a `List(*void)`, and iterating that list to feed each
+// handle back through C.
+//
+// No new ABI shape — pointer-in, pointer-out. The lemma locked in:
+// handle-shaped flows survive sx's struct-field assignment, List
+// storage, and iteration-then-call cycles.
+
+#import "modules/std.sx";
+
+#import c {
+    #include "ffi-09-foreign-result-chain.h";
+    #source "ffi-09-foreign-result-chain.c";
+};
+
+// Struct field hosts an FFI-returned handle.
+Counter :: struct {
+    handle: *void = null;
+    label: string;
+}
+
+main :: () -> s32 {
+    // ── 1. Chain: make → bump → peek ───────────────────────────────
+    a := ffi_chain_make(100);
+    print("peek after make    = {}\n", ffi_chain_peek(a));
+    print("bump(+5)           = {}\n", ffi_chain_bump(a, 5));
+    print("bump(+3)           = {}\n", ffi_chain_bump(a, 3));
+    print("peek after bumps   = {}\n", ffi_chain_peek(a));
+
+    // ── 2. Stash handle in a struct field, use through `.handle` ──
+    c : Counter = .{ handle = ffi_chain_make(50), label = "ctr-a" };
+    print("ctr label          = {}\n", c.label);
+    print("ctr peek           = {}\n", ffi_chain_peek(c.handle));
+    ffi_chain_bump(c.handle, 7);
+    print("ctr after bump     = {}\n", ffi_chain_peek(c.handle));
+
+    // ── 3. Push handles into a List, iterate, feed back to C ──────
+    handles : List(*void) = .{};
+    i : s32 = 0;
+    while i < 3 {
+        h := ffi_chain_make(i * 10);
+        handles.append(h);
+        i += 1;
+    }
+
+    j : s64 = 0;
+    while j < handles.len {
+        h := handles.items[j];
+        v := ffi_chain_peek(h);
+        print("list[{}] peek      = {}\n", j, v);
+        j += 1;
+    }
+
+    // Iterate again, bump each, observe the cumulative effect.
+    j = 0;
+    while j < handles.len {
+        ffi_chain_bump(handles.items[j], 1);
+        j += 1;
+    }
+    j = 0;
+    while j < handles.len {
+        print("list[{}] after bump= {}\n", j, ffi_chain_peek(handles.items[j]));
+        j += 1;
+    }
+
+    // ── Cleanup ─────────────────────────────────────────────────────
+    ffi_chain_dispose(a);
+    ffi_chain_dispose(c.handle);
+    j = 0;
+    while j < handles.len {
+        ffi_chain_dispose(handles.items[j]);
+        j += 1;
+    }
+
+    0;
+}

tests/expected/ffi-09-foreign-result-chain.exit

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

tests/expected/ffi-09-foreign-result-chain.txt

@@ -0,0 +1,13 @@
+peek after make    = 100
+bump(+5)           = 105
+bump(+3)           = 108
+peek after bumps   = 108
+ctr label          = ctr-a
+ctr peek           = 50
+ctr after bump     = 57
+list[0] peek      = 0
+list[1] peek      = 10
+list[2] peek      = 20
+list[0] after bump= 1
+list[1] after bump= 11
+list[2] after bump= 21