fix: resolve race condition in nsld.sh with parallel linker invocations

Xcode can invoke nsld.sh concurrently for different architectures during
incremental builds. All invocations shared a single Swift-Modules directory,
causing EINVAL errors on APFS when one process deletes the directory while
another is writing module.modulemap.

Use per-architecture directories (Swift-Modules-$TARGET_ARCH) so parallel
invocations no longer collide. Export the arch-specific path into
HEADER_SEARCH_PATHS so the metadata generator can still locate the modulemap.

Author: NathanWalker

project-template-ios/internal/nsld.sh

@@ -1,12 +1,6 @@
 #!/usr/bin/env bash
 source ./.build_env_vars.sh
 
-MODULES_DIR="$SRCROOT/internal/Swift-Modules"
-
-function DELETE_SWIFT_MODULES_DIR() {
-    rm -rf "$MODULES_DIR"
-}
-
 function getArch() {
     while [[ $# -gt 0 ]]
     do
@@ -24,6 +18,16 @@ function getArch() {
     done
 }
 
+# Workaround for ARCH being set to `undefined_arch` here. Extract it from command line arguments.
+TARGET_ARCH=$(getArch "$@")
+
+# Use per-architecture directory to avoid race conditions with parallel linker invocations
+MODULES_DIR="$SRCROOT/internal/Swift-Modules-$TARGET_ARCH"
+
+function DELETE_SWIFT_MODULES_DIR() {
+    rm -rf "$MODULES_DIR"
+}
+
 function GEN_MODULEMAP() {
     ARCH_ARG=$1
     SWIFT_HEADER_DIR=$PER_VARIANT_OBJECT_FILE_DIR/$ARCH_ARG
@@ -52,9 +56,8 @@ function GEN_METADATA() {
     popd
 }
 
-# Workaround for ARCH being set to `undefined_arch` here. Extract it from command line arguments.
-TARGET_ARCH=$(getArch "$@")
 GEN_MODULEMAP $TARGET_ARCH
+export HEADER_SEARCH_PATHS="$HEADER_SEARCH_PATHS $MODULES_DIR"
 printf "Generating metadata..."
 GEN_METADATA $TARGET_ARCH
 DELETE_SWIFT_MODULES_DIR

project-template-vision/internal/nsld.sh

@@ -1,12 +1,6 @@
 #!/usr/bin/env bash
 source ./.build_env_vars.sh
 
-MODULES_DIR="$SRCROOT/internal/Swift-Modules"
-
-function DELETE_SWIFT_MODULES_DIR() {
-    rm -rf "$MODULES_DIR"
-}
-
 function getArch() {
     while [[ $# -gt 0 ]]
     do
@@ -24,6 +18,16 @@ function getArch() {
     done
 }
 
+# Workaround for ARCH being set to `undefined_arch` here. Extract it from command line arguments.
+TARGET_ARCH=$(getArch "$@")
+
+# Use per-architecture directory to avoid race conditions with parallel linker invocations
+MODULES_DIR="$SRCROOT/internal/Swift-Modules-$TARGET_ARCH"
+
+function DELETE_SWIFT_MODULES_DIR() {
+    rm -rf "$MODULES_DIR"
+}
+
 function GEN_MODULEMAP() {
     ARCH_ARG=$1
     SWIFT_HEADER_DIR=$PER_VARIANT_OBJECT_FILE_DIR/$ARCH_ARG
@@ -52,9 +56,8 @@ function GEN_METADATA() {
     popd
 }
 
-# Workaround for ARCH being set to `undefined_arch` here. Extract it from command line arguments.
-TARGET_ARCH=$(getArch "$@")
 GEN_MODULEMAP $TARGET_ARCH
+export HEADER_SEARCH_PATHS="$HEADER_SEARCH_PATHS $MODULES_DIR"
 printf "Generating metadata..."
 GEN_METADATA $TARGET_ARCH
 DELETE_SWIFT_MODULES_DIR