feat(cli): add --watch mode to convert (#39)

Poll source files/directories and regenerate output on change.
Output path is excluded from watching; errors don't stop the loop.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

cmd/relspec/watch_test.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestWatchSnapshotExcludesOutput(t *testing.T) {
+	dir := t.TempDir()
+	out := filepath.Join(dir, "out")
+	if err := os.MkdirAll(out, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	src := filepath.Join(dir, "schema.dbml")
+	if err := os.WriteFile(src, []byte("a"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	before := takeWatchSnapshot([]string{dir}, out)
+	if err := os.WriteFile(filepath.Join(out, "gen.go"), []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if after := takeWatchSnapshot([]string{dir}, out); !before.equal(after) {
+		t.Errorf("writing into the excluded output path changed the snapshot")
+	}
+	if err := os.WriteFile(src, []byte("changed"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if after := takeWatchSnapshot([]string{dir}, out); before.equal(after) {
+		t.Errorf("modifying a source file did not change the snapshot")
+	}
+}
+
+func TestWatchLoopRerunsOnChange(t *testing.T) {
+	dir := t.TempDir()
+	src := filepath.Join(dir, "schema.dbml")
+	if err := os.WriteFile(src, []byte("a"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	var runs atomic.Int32
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		watchLoop(ctx, io.Discard, []string{src}, "", 10*time.Millisecond, func() error {
+			runs.Add(1)
+			return nil
+		})
+	}()
+
+	waitFor(t, func() bool { return runs.Load() == 1 })
+	if err := os.WriteFile(src, []byte("changed content"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	waitFor(t, func() bool { return runs.Load() == 2 })
+	cancel()
+	<-done
+}
+
+func TestRunConvertWatchValidation(t *testing.T) {
+	oldPath, oldList, oldDry, oldInt := convertSourcePath, convertFromList, convertDryRun, convertWatchInterval
+	defer func() {
+		convertSourcePath, convertFromList, convertDryRun, convertWatchInterval = oldPath, oldList, oldDry, oldInt
+	}()
+	convertSourcePath, convertFromList, convertDryRun, convertWatchInterval = "", nil, false, time.Second
+	if err := runConvertWatch(context.Background(), &bytes.Buffer{}, nil); err == nil {
+		t.Error("expected error without --from-path/--from-list")
+	}
+	convertSourcePath, convertDryRun = "x.dbml", true
+	if err := runConvertWatch(context.Background(), &bytes.Buffer{}, nil); err == nil {
+		t.Error("expected error combining --watch with --dry-run")
+	}
+}
+
+func waitFor(t *testing.T, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatal("condition not met in time")
+}