Fix macOS framework layout so the inner bundle can be signed (#39)

The signing job failed on every python-macos-dart archive with:

  Python.framework: unsealed contents present in the root directory of
  an embedded framework

macOS frameworks are VERSIONED bundles: everything at the framework root must be
a symlink into Versions/Current, and only `Versions` itself may be a real
directory. package-macos-for-dart.sh wrote the overlaid `Modules` and the public
`Headers` as real directories at the root. That was invisible while only the
outer xcframework was signed -- the outer seal does not care -- and became fatal
the moment the inner framework was signed in its own right.

They now go into Versions/Current with symlinks at the root, which is both what
codesign requires and what CPython's own macOS framework already does for Headers
and Resources. Consumers resolve Headers/ and Modules/ through the symlinks
exactly as before.

Also fixes the verifier, which was wrong in a way the same run exposed: it probed
for `_CodeSignature` or `Versions/A/_CodeSignature`, but a versioned bundle's
version directory is not always "A" -- CPython's macOS framework uses
Versions/3.14 -- so a correctly signed macOS framework was reported as unsigned.
It now asks `codesign -dv`, which is layout-agnostic and is the actual question.

Verified with the real identity against the released 20260729 macOS archive:
inner and outer both sign and verify, dart_bridge v1.7.1 (Versions/A) still
verifies, and stripping the inner signature still fails the run.

Author: FeodorFitsner

darwin/package-macos-for-dart.sh

@@ -34,12 +34,34 @@ mkdir -p $stdlib_dir
 
 # copy Python.xcframework
 rsync -av --exclude-from=$script_dir/python-darwin-framework.exclude $python_apple_support_root/support/$python_version_short/macOS/Python.xcframework $frameworks_dir
-cp -r $script_dir/Modules $frameworks_dir/Python.xcframework/macos-arm64_x86_64/Python.framework
-mkdir -p $frameworks_dir/Python.xcframework/macos-arm64_x86_64/Python.framework/Headers
-cp -r $python_apple_support_root/support/$python_version_short/macOS/Python.xcframework/macos-arm64_x86_64/Python.framework/Versions/$python_version_short/include/python$python_version_short/* $frameworks_dir/Python.xcframework/macos-arm64_x86_64/Python.framework/Headers
+# Overlay the module map and public headers.
+#
+# macOS frameworks are VERSIONED bundles: everything at the framework root must be
+# a symlink into Versions/Current, and only `Versions` itself is a real directory.
+# These used to be written as real directories at the root, which codesign rejects
+# outright once the framework is signed in its own right:
+#
+#   Python.framework: unsealed contents present in the root directory of an
+#   embedded framework
+#
+# That went unnoticed while only the outer xcframework was signed. Write them into
+# Versions/Current and symlink from the root, which is both what codesign requires
+# and what CPython's own macOS framework does for Headers and Resources.
+macos_fw=$frameworks_dir/Python.xcframework/macos-arm64_x86_64/Python.framework
+[ -d "$macos_fw/Versions/Current" ] || {
+    echo "expected a versioned macOS framework at $macos_fw"; exit 1; }
+
+cp -r $script_dir/Modules "$macos_fw/Versions/Current/"
+mkdir -p "$macos_fw/Versions/Current/Headers"
+cp -r $python_apple_support_root/support/$python_version_short/macOS/Python.xcframework/macos-arm64_x86_64/Python.framework/Versions/$python_version_short/include/python$python_version_short/* "$macos_fw/Versions/Current/Headers"
 # The built-in modulemap (if the source framework shipped one) is replaced by the
 # overlaid darwin/Modules/module.modulemap above; -f tolerates builds without one.
-rm -f $frameworks_dir/Python.xcframework/macos-arm64_x86_64/Python.framework/Headers/module.modulemap
+rm -f "$macos_fw/Versions/Current/Headers/module.modulemap"
+
+for _d in Headers Modules; do
+    rm -rf "$macos_fw/$_d"
+    ln -sfn "Versions/Current/$_d" "$macos_fw/$_d"
+done
 
 # Last mutation of the bundle: stable, provider-owned identifier for the Python
 # runtime, replacing CPython's shared `org.python.python`. The macOS framework is

darwin/xcframework_signing.sh

@@ -285,7 +285,12 @@ xcf_verify_one() {
     local fw count=0
     while IFS= read -r fw; do
         [ -n "$fw" ] || continue
-        if [ ! -d "$fw/_CodeSignature" ] && [ ! -d "$fw/Versions/A/_CodeSignature" ]; then
+        # `codesign -dv` rather than probing for a _CodeSignature directory: a
+        # versioned bundle keeps it at Versions/<name>/_CodeSignature, and the
+        # version directory is not always "A" (CPython's macOS framework uses the
+        # Python version, e.g. Versions/3.14). codesign exits non-zero with
+        # "code object is not signed at all", which is the question being asked.
+        if ! codesign -dv "$fw" >/dev/null 2>&1; then
             xcf_err "$fw: inner framework is unsigned"
             return 1
         fi