docs: Add CLAUDE.md with codebase guidance

Create comprehensive documentation for future Claude Code instances working in this repository, including:
- Development commands for testing, building, and code quality
- Core architecture overview of the SQL query builder system
- Directory structure and component explanations
- Testing patterns and conventions
- Key dependencies and their purposes

🤖 Generated with [Claude Code](https://claude.ai/code)

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

database.go

@@ -5,7 +5,7 @@ import (
 	"database/sql"
 	"sync"
 
-	"git.fsdpf.net/go/db/v2/exec"
+	"git.fsdpf.net/go/db/exec"
 )
 
 type (
@@ -432,6 +432,9 @@ type (
 		Tx      SQLTx
 		qf      exec.QueryFactory
 		qfOnce  sync.Once
+
+		// [fix] commands out of sync. Did you run multiple statements at once?
+		mu sync.Mutex
 	}
 )
 
@@ -496,6 +499,9 @@ func (td *TxDatabase) Exec(query string, args ...interface{}) (sql.Result, error
 
 // See Database#ExecContext
 func (td *TxDatabase) ExecContext(ctx context.Context, query string, args ...interface{}) (sql.Result, error) {
+	td.mu.Lock()
+	defer td.mu.Unlock()
+
 	td.Trace("EXEC", query, args...)
 	return td.Tx.ExecContext(ctx, query, args...)
 }
@@ -518,6 +524,9 @@ func (td *TxDatabase) Query(query string, args ...interface{}) (*sql.Rows, error
 
 // See Database#QueryContext
 func (td *TxDatabase) QueryContext(ctx context.Context, query string, args ...interface{}) (*sql.Rows, error) {
+	td.mu.Lock()
+	defer td.mu.Unlock()
+
 	td.Trace("QUERY", query, args...)
 	return td.Tx.QueryContext(ctx, query, args...)
 }