bundling: fs/process stdlib + post-link callback + Apple .app in sx

Campaign Weeks 3-6 of /Users/agra/.claude/plans/lets-plan-to-move-splendid-pumpkin.md
land in one push: the bundling pipeline that used to live in
src/target.zig (createBundle, embedFramework, extractEntitlements,
buildInfoPlist, codesign) now lives in
library/modules/platform/bundle.sx and runs in the IR interpreter
after target.link() returns.

New language-side surface:
- library/modules/fs.sx — POSIX libc bindings (open/read/write/close,
  mkdir/unlink/rmdir, chmod, rename, access, basename/dirname). Variadic
  open() lowers to C's varargs via the new args: ..T form. Direct libc
  calls bypass *File method dispatch so they work from the post-link
  IR interpreter.
- library/modules/process.sx — popen-based run(cmd) returning
  ProcessResult{ exit_code, stdout }, plus env() and find_executable().
- library/modules/std.sx — xml_escape(s) and variadic path_join(parts).
- library/modules/compiler.sx — BuildOptions grows
  set_post_link_callback / set_post_link_module / binary_path
  accessors; bundle_path/bundle_id/codesign_identity/provisioning_profile
  setters + accessors; per-target predicates is_macos/is_ios/
  is_ios_device/is_ios_simulator + target_triple; framework_count /
  framework_at(i) / framework_path_count / framework_path_at(i);
  add_asset_dir(src, dest) + asset_dir_count / src_at / dest_at.

Compiler-side wiring:
- src/ir/compiler_hooks.zig — BuildConfig now carries post_link_callback_fn,
  post_link_module, binary_path, bundle_*, target_triple,
  target_frameworks, target_framework_paths, asset_dirs. Hook registry
  exposes every accessor; getters return "" / 0 for unset fields so
  bundle.sx can treat absent values uniformly.
- src/ir/host_ffi.zig (new) — dlsym(RTLD_DEFAULT) + arity-switched cdecl
  trampolines so #foreign("c") declarations resolve through the host
  libc during #run / post-link interpretation.
- src/ir/interp.zig — callForeign dispatch; build_config pointer
  injection so accessor hooks see live state during re-entry.
- src/core.zig — keeps the IR module alive past generateCode; exposes
  invokeByName / invokeByFuncId so main.zig can re-enter the
  interpreter after linking.
- src/main.zig — wires bundle/codesign/provisioning CLI flags +
  target_triple + framework lists into BuildConfig; invokes the
  post-link callback (by FuncId or by <module>.bundle_main lookup) once
  target.link() returns. When --bundle is set but no callback is
  registered, auto-falls-back to post_link_module = "platform.bundle"
  so the legacy --bundle CLI keeps working for any program that imports
  modules/platform/bundle.sx.

Apple .app bundler (library/modules/platform/bundle.sx):
- Single bundle_main entry covers macOS, iOS simulator, iOS device.
  Per-target Info.plist switch keys off is_ios()/is_ios_simulator() —
  iOS emits UIDeviceFamily / LSRequiresIPhoneOS /
  UIApplicationSceneManifest / DTPlatformName (iPhoneOS or
  iPhoneSimulator); macOS emits the minimal CFBundle* set.
- iOS-only steps:
  - Provisioning embed: fs.read_file + fs.write_file to
    <bundle>/embedded.mobileprovision.
  - Framework embed: recursive cp -R per -F search path into
    <bundle>/Frameworks/<Name>.framework/ (until fs.sx grows list_dir).
  - Entitlements extraction: four process.run calls (security cms -D,
    plutil -extract Entitlements xml1, plutil -extract
    ApplicationIdentifierPrefix.0, plutil -replace application-identifier)
    resolving the wildcard <TEAM>.* -> <TEAM>.<bundle_id>.
  - Real codesign with --entitlements when present.
- Asset dirs (add_asset_dir): recursive cp -R src/. into <bundle>/dest/.
  Missing src is treated as "nothing to do" so projects can register
  add_asset_dir("assets", "assets") unconditionally.

Parser:
- parseStmt() now accepts #import \"path\"; and #framework \"Name\"; as
  statement-position tokens. Needed for top-level
  inline if OS == .android { #import \"modules/platform/android.sx\"; }
  blocks (issue-0042 flatten pass surfaces them); chess's
  inline-if-with-#import was rejected at parse time before this fix.

Removals from src/target.zig:
- createBundle, embedFramework, extractEntitlements, buildInfoPlist,
  codesign (~210 lines). main.zig no longer calls createBundle after
  link(); the sx callback is the single entry point.

Tests / regression markers (all run under sx run host JIT):
- examples/115-post-link-callback.sx — callback registration round-trip.
- examples/116-fs-roundtrip.sx — fs.write_file -> fs.read_file -> exists.
- examples/117-process-roundtrip.sx — process.run + env + find_executable.
- examples/118-macos-bundle.sx — macOS .app via bundle_main callback.
- examples/119-interp-cast-ptr-cmp.sx — cast(T) val under interpreter.
- examples/120-interp-variadic-any.sx — variadic ..Any indexing in IR
  interpreter.
- examples/121-ios-sim-bundle.sx — iOS-sim cross-compile + .app with
  iOS-shaped Info.plist (added to tests/cross_compile.sh as the
  ios-sim tuple).
- examples/122-ios-device-bundle.sx — iOS device cross-compile +
  full codesign pipeline (provisioning embed + entitlements
  extraction + --entitlements codesign). Manually verified end-to-end:
  installed via xcrun devicectl device install app + launched
  successfully on iPhone 17 Pro.
- examples/123-inline-if-import-in-body.sx — locks in the parser fix.

zig build && zig build test && bash tests/run_examples.sh => 141 passed,
0 failed; bash tests/cross_compile.sh => 7 passed, 0 failed.

library/modules/std.sx

@@ -152,6 +152,67 @@ substr :: (s: string, start: s64, len: s64) -> string {
     buf;
 }
 
+// Replace XML special characters with their entity references. Used
+// when emitting Info.plist / AndroidManifest content from sx values
+// that may contain user-supplied text (bundle id, app name, etc).
+xml_escape :: (s: string) -> string {
+    result := "";
+    i := 0;
+    seg_start := 0;
+    while i < s.len {
+        c := s[i];
+        // 38='&', 60='<', 62='>', 34='"', 39='\''
+        ent := "";
+        if c == 38 { ent = "&amp;"; }
+        if c == 60 { ent = "&lt;"; }
+        if c == 62 { ent = "&gt;"; }
+        if c == 34 { ent = "&quot;"; }
+        if c == 39 { ent = "&apos;"; }
+        if ent.len > 0 {
+            if i > seg_start {
+                result = concat(result, substr(s, seg_start, i - seg_start));
+            }
+            result = concat(result, ent);
+            seg_start = i + 1;
+        }
+        i += 1;
+    }
+    if seg_start < s.len {
+        result = concat(result, substr(s, seg_start, s.len - seg_start));
+    }
+    result;
+}
+
+// Join path components with the POSIX separator ('/'). Skips empty
+// components and collapses duplicate separators at component
+// boundaries. Used for bundle paths where Apple .app and Android APK
+// both expect POSIX-style paths.
+path_join :: (parts: ..string) -> string {
+    result := "";
+    i := 0;
+    while i < parts.len {
+        p := parts[i];
+        if p.len > 0 {
+            if result.len > 0 {
+                tail := result[result.len - 1];
+                head := p[0];
+                if tail == 47 {
+                    if head == 47 {
+                        p = substr(p, 1, p.len - 1);
+                    }
+                } else {
+                    if head != 47 {
+                        result = concat(result, "/");
+                    }
+                }
+            }
+            result = concat(result, p);
+        }
+        i += 1;
+    }
+    result;
+}
+
 struct_to_string :: (s: $T) -> string {
     result := concat(type_name(T), "{");
     i := 0;