fix(ios): show screen-audio broadcast extension in picker

The broadcast upload extension never appeared in the iOS broadcast picker
(or Control Center screen-recording list) because its Info.plist set
RPBroadcastProcessMode to the invalid string "BroadcastUpload". iOS only
recognises "RPBroadcastProcessModeSampleBuffer" for a sample-buffer upload
extension and silently rejects any other value, hiding the extension.

Also sign VoiceCatCore.xcframework in build-xcframework.sh: Xcode 15+ rejects
unsigned binary XCFrameworks consumed as SwiftPM binary targets. Signs with
CODESIGN_IDENTITY, else the first Apple Development identity, else ad-hoc.

clients/apple/scripts/build-xcframework.sh

@@ -155,5 +155,20 @@ xcodebuild -create-xcframework "${args[@]}" -output "$OUTPUT"
 # Clean up staged headers (the xcframework has its own copy now).
 rm -rf "$STAGED_ROOT"
 
+# ── Code-sign ────────────────────────────────────────────────────────────────────
+# Xcode 15+ rejects unsigned binary XCFrameworks consumed as SwiftPM binary targets
+# ("The Framework VoiceCatCore.xcframework is unsigned."). Sign with CODESIGN_IDENTITY
+# if set, else the first "Apple Development" identity in the keychain, else ad-hoc ("-").
+# The .xcframework is a local build artifact (gitignored), so signing with the local
+# developer identity is fine and not committed.
+SIGN_ID="${CODESIGN_IDENTITY:-}"
+if [[ -z "$SIGN_ID" ]]; then
+    SIGN_ID="$(security find-identity -v -p codesigning 2>/dev/null \
+                | awk '/Apple Development/{print $2; exit}')"
+fi
+SIGN_ID="${SIGN_ID:--}"   # fall back to ad-hoc
+codesign --force --timestamp=none --sign "$SIGN_ID" "$OUTPUT"
+echo "[build-xcframework] signed with identity: $SIGN_ID"
+
 echo "[build-xcframework] done -> $OUTPUT"
 xcodebuild -list -xcframework "$OUTPUT" 2>/dev/null || true