perf: use CoW (copy-on-write) cloning for directory copies

Add _fast_copy_dir() that leverages filesystem-level cloning
(macOS APFS cp -c, Linux cp --reflink=auto) for near-instant
directory copies on supported filesystems, with automatic
fallback to standard cp -RP on ext4/NTFS/others.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: arwtyxouymz

lib/copy.sh

@@ -75,6 +75,32 @@ merge_copy_patterns() {
   fi
 }
 
+# Copy a directory using CoW (copy-on-write) when available, falling back to standard cp.
+# macOS APFS: cp -cRP (clone); Linux Btrfs/XFS: cp --reflink=auto -RP
+# Callers must guard the return value with `if` or `|| true` (set -e safe).
+# Usage: _fast_copy_dir src dest
+_fast_copy_dir() {
+  local src="$1" dest="$2"
+  local os
+  os=$(detect_os)
+
+  case "$os" in
+    darwin)
+      # Try CoW clone first; if unsupported, fall back to regular copy
+      if cp -cRP "$src" "$dest" 2>/dev/null; then
+        return 0
+      fi
+      cp -RP "$src" "$dest"
+      ;;
+    linux)
+      cp --reflink=auto -RP "$src" "$dest"
+      ;;
+    *)
+      cp -RP "$src" "$dest"
+      ;;
+  esac
+}
+
 # Copy a single file to destination, handling exclusion, path preservation, and dry-run
 # Usage: _copy_pattern_file file dst_root excludes preserve_paths dry_run
 # Returns: 0 if file was copied (or would be in dry-run), 1 if skipped/failed
@@ -307,8 +333,8 @@ copy_directories() {
       dest_parent=$(dirname "$dest_dir")
       mkdir -p "$dest_parent"
 
-      # Copy directory (cp -RP preserves symlinks as symlinks)
-      if cp -RP "$dir_path" "$dest_parent/" 2>/dev/null; then
+      # Copy directory using CoW when available (preserves symlinks as symlinks)
+      if _fast_copy_dir "$dir_path" "$dest_parent/"; then
         log_info "Copied directory $dir_path"
         copied_count=$((copied_count + 1))
         _apply_directory_excludes "$dest_parent" "$dir_path" "$excludes"

tests/copy_safety.bats

@@ -2,6 +2,7 @@
 
 setup() {
   load test_helper
+  source "$PROJECT_ROOT/lib/platform.sh"
   source "$PROJECT_ROOT/lib/copy.sh"
 }
 
@@ -82,3 +83,41 @@ setup() {
   excludes=$(printf '%s\n' "*.log" "dist/*")
   ! is_excluded "src/app.js" "$excludes"
 }
+
+# --- _fast_copy_dir tests ---
+
+@test "_fast_copy_dir copies directory contents" {
+  local src dst
+  src=$(mktemp -d)
+  dst=$(mktemp -d)
+  mkdir -p "$src/mydir/sub"
+  echo "hello" > "$src/mydir/sub/file.txt"
+
+  _fast_copy_dir "$src/mydir" "$dst/"
+
+  [ -f "$dst/mydir/sub/file.txt" ]
+  [ "$(cat "$dst/mydir/sub/file.txt")" = "hello" ]
+  rm -rf "$src" "$dst"
+}
+
+@test "_fast_copy_dir preserves symlinks" {
+  local src dst
+  src=$(mktemp -d)
+  dst=$(mktemp -d)
+  mkdir -p "$src/mydir"
+  echo "target" > "$src/mydir/real.txt"
+  ln -s real.txt "$src/mydir/link.txt"
+
+  _fast_copy_dir "$src/mydir" "$dst/"
+
+  [ -L "$dst/mydir/link.txt" ]
+  [ "$(readlink "$dst/mydir/link.txt")" = "real.txt" ]
+  rm -rf "$src" "$dst"
+}
+
+@test "_fast_copy_dir fails on nonexistent source" {
+  local dst
+  dst=$(mktemp -d)
+  ! _fast_copy_dir "/nonexistent/path" "$dst/"
+  rm -rf "$dst"
+}