Native iPhone port of Factorio 2.0.77

Run the original macOS ARM64 game code on iOS:
- tools/retarget.py: header-only Mach-O retargeting to an iOS dylib
- platform/native: UIKit host and macOS API shims (AppKit for SDL's
  Cocoa backend, CoreGraphics displays, CoreAudio device, Metal device
  properties, Wuffs PNG decoding, Steam and peripheral stubs)
- touch, overlay, iOS keyboard and hardware keyboard input
- tools/build_native.py: build, sign, install, data copy, launch
- docs: build guide, controls, architecture

Also includes the earlier static-translation research (ARM64 -> WASM ->
native) under docs/research, runtime/, tests/ and tools/.

No game files are included; the build reads them from a local install.

tests/save_prefix_probe.c

@@ -0,0 +1,24 @@
+#include "save_header.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(int argc, char **argv) {
+  if (argc != 3) return 2;
+  char *end = NULL;
+  unsigned long requested = strtoul(argv[2], &end, 10);
+  if (!end || *end || requested > 1024 * 1024 + 1) return 2;
+  uint8_t *guarded = malloc(requested + 16);
+  if (!guarded) return 2;
+  memset(guarded, 0xa5, requested + 16);
+  int status = factorio_save_prefix(argv[1], guarded + 8, requested);
+  for (unsigned i=0;i<8;++i) {
+    if (guarded[i] != 0xa5 || guarded[requested+8+i] != 0xa5) return 3;
+  }
+  if (status) for (unsigned long i=0;i<requested;++i) if (guarded[i+8]!=0xa5) return 4;
+  printf("{\"status\":%d,\"prefix_hex\":\"", status);
+  if (!status) for (unsigned long i=0;i<requested;++i) printf("%02x",guarded[i+8]);
+  puts("\"}");
+  free(guarded);
+  return status ? 1 : 0;
+}