feat(spm): auto-sync detects added/removed native source files

The Xcode auto-sync build phase previously only watched package.json,
react-native.config.js, lockfiles, and node_modules mtimes. Adding or
removing a source file inside an spm.modules entry or an autolinked
dep was invisible — adds were silently skipped (missing from the
sources: allowlist) and removes broke the build (allowlist pointed at
a vanished file).

The autolinker now emits build/generated/autolinking/.spm-sync-watch-paths,
a deduped list of every module's source dir it consumed. The build-phase
shell extends its staleness checks with `find -newer "$STAMP"` against
each watched dir. POSIX directory mtimes update on both add and remove,
so a single check covers both cases. Sub-millisecond per dir; the
fast path is unchanged when nothing has changed.

When triggered, the existing sync runs the autolinker, which regenerates
the wrapper Package.swift files and their sources: allowlists.

Author: chrfalch

packages/react-native/scripts/spm/generate-spm-autolinking.js

@@ -1296,6 +1296,20 @@ function main(argv /*:: ?: Array<string> */) /*: void */ {
   fs.writeFileSync(outputPath, aggregatorContent, 'utf8');
   log(`Generated: ${path.relative(appRoot, outputPath)}`);
 
+  // .spm-sync-watch-paths: absolute paths the Xcode auto-sync build phase
+  // should watch for add/remove of native source files. Updating the dir
+  // mtime via touch/rm of a child file is enough to invalidate the stamp,
+  // so the sync re-runs and the `sources:` allowlist is regenerated.
+  // Each module's source dir is already known via entryAbsDirs.
+  const watchPaths = Array.from(new Set(entryAbsDirs.values()))
+    .filter(p => p.length > 0 && fs.existsSync(p))
+    .sort();
+  fs.writeFileSync(
+    path.join(outputDir, '.spm-sync-watch-paths'),
+    watchPaths.join('\n') + (watchPaths.length > 0 ? '\n' : ''),
+    'utf8',
+  );
+
   // AutolinkedAggregate is glue; needs at least one source file (Swift, so we
   // sidestep the Obj-C public-headers-dir requirement).
   const aggregateDir = path.join(outputDir, 'AutolinkedAggregate');

packages/react-native/scripts/spm/generate-spm-xcodeproj.js

@@ -632,6 +632,20 @@ if [ "$STALE" -eq 0 ] && [ "$SRCROOT" != "$PROJECT_ROOT" ]; then
   fi
 fi
 
+# Check 1.5: watched module source dirs (catches add/remove of source files
+# in spm.modules and autolinked deps). Directory mtime updates on both add
+# and remove of children, so a single -newer check covers both cases.
+WATCH_FILE="$SRCROOT/build/generated/autolinking/.spm-sync-watch-paths"
+if [ "$STALE" -eq 0 ] && [ -f "$WATCH_FILE" ]; then
+  while IFS= read -r DIR; do
+    [ -z "$DIR" ] && continue
+    if [ -d "$DIR" ] && [ -n "$(find "$DIR" -newer "$STAMP" -print -quit 2>/dev/null)" ]; then
+      STALE=1
+      break
+    fi
+  done < "$WATCH_FILE"
+fi
+
 # Check 2: codegen spec files changed via git (covers monorepo after git pull)
 if [ "$STALE" -eq 0 ] && [ -f "$STAMP" ]; then
   STAMP_TIME=$(stat -f %m "$STAMP" 2>/dev/null || stat -c %Y "$STAMP" 2>/dev/null || echo 0)