Add Windows native watcher with Unix socket IPC

Architecture:
- Windows binary: watches files via ReadDirectoryChangesW (fsnotify),
  sends events as newline-delimited JSON over Unix domain socket
- Container: performs initial sync, then listens on socket for events
  from Windows watcher

New files:
- cmd_windows.go: Windows serve command (watcher + socket server)
- cmd_linux.go: Linux socket client + event listener
- types.go: shared FileEvent, FileMeta, Manifest types
- watcher_test_helper.go: inotify-based watcher for integration tests

Changes:
- main.go: simplified to just call run()
- watcher.go: removed fsnotify dependency, extracted Watcher interface
- sync.go: removed duplicate type definitions
- watcher_test.go: uses watchForTests() helper

Usage:
  sourcewatch.exe -dir C:\myproject -socket D:\sourcewatch.sock
  docker run -v D:\sourcewatch.sock:/run/sourcewatch.sock \
             -v C:\myproject:/host:ro -v mydata:/data sourcewatch

1 files changed, 142 insertions, 0 deletions

diff --git a/watcher_test_helper.go b/watcher_test_helper.go
new file mode 100644
index 0000000..519d3f6
--- /dev/null
+++ b/watcher_test_helper.go
@@ -0,0 +1,142 @@
+//go:build !windows
+
+package main
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// watchForTests uses inotify directly for integration testing.
+// This is only used in tests - production uses socket-based events from Windows watcher.
+func watchForTests(ctx context.Context, hostDir, dataDir string, manifest *Manifest, stack *GitignoreStack, dryRun bool) error {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return err
+	}
+	defer watcher.Close()
+
+	var mu sync.Mutex
+	debounceTimers := make(map[string]*time.Timer)
+	debounceOps := make(map[string]fsnotify.Op)
+
+	if err := addWatchersForTests(watcher, hostDir, stack); err != nil {
+		return err
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return nil
+
+		case event, ok := <-watcher.Events:
+			if !ok {
+				return nil
+			}
+
+			relPath, err := filepath.Rel(hostDir, event.Name)
+			if err != nil {
+				continue
+			}
+
+			if isGitDir(relPath) {
+				continue
+			}
+
+			if stack.IsIgnored(relPath) {
+				continue
+			}
+
+			if event.Op&fsnotify.Create != 0 {
+				srcPath := filepath.Join(hostDir, relPath)
+				info, err := os.Lstat(srcPath)
+				if err == nil && info.IsDir() {
+					if hasGitignore(srcPath) {
+						stack.Push(srcPath)
+					}
+					watcher.Add(srcPath)
+					filepath.Walk(srcPath, func(path string, info os.FileInfo, err error) error {
+						if err != nil || !info.IsDir() || path == srcPath {
+							return nil
+						}
+						watcher.Add(path)
+						return nil
+					})
+				}
+			}
+
+			mu.Lock()
+			if existing, exists := debounceOps[relPath]; exists {
+				debounceTimers[relPath].Stop()
+				debounceOps[relPath] = existing | event.Op
+			} else {
+				debounceOps[relPath] = event.Op
+			}
+			path := relPath
+			debounceTimers[relPath] = time.AfterFunc(200*time.Millisecond, func() {
+				mu.Lock()
+				op := debounceOps[path]
+				delete(debounceOps, path)
+				delete(debounceTimers, path)
+				mu.Unlock()
+
+				destPath := filepath.Join(dataDir, path)
+				srcPath := filepath.Join(hostDir, path)
+
+				switch {
+				case op&fsnotify.Create != 0:
+					syncCreate(srcPath, destPath, path, manifest, stack, dryRun, watcher)
+				case op&fsnotify.Write != 0:
+					syncWrite(srcPath, destPath, path, manifest, dryRun)
+				case op&fsnotify.Remove != 0:
+					syncRemove(destPath, path, manifest, dryRun)
+				case op&fsnotify.Rename != 0:
+					syncRemove(destPath, path, manifest, dryRun)
+				}
+			})
+			mu.Unlock()
+
+		case err, ok := <-watcher.Errors:
+			if !ok {
+				return nil
+			}
+			_ = err
+		}
+	}
+}
+
+func addWatchersForTests(watcher *fsnotify.Watcher, root string, stack *GitignoreStack) error {
+	return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		relPath, err := filepath.Rel(root, path)
+		if err != nil {
+			return err
+		}
+
+		if isGitDir(relPath) {
+			if info.IsDir() {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+
+		if info.IsDir() {
+			if hasGitignore(path) {
+				stack.Push(path)
+			}
+			if !stack.IsIgnored(relPath) {
+				return watcher.Add(path)
+			}
+		}
+
+		return nil
+	})
+}