Escape the database path in the SQLite connection string (closes #55)

openDB put the path into the connection string unescaped, so a ? or #
in it ended the file name and a % started an escape: scan could
silently fill a database under a shortened name. The path now goes
through net/url as a file: URI. An absolute path gets an empty host and
a relative path none, because SQLite reads what follows file:// up to
the next slash as a host name. The path is not cleaned, so it stays
exactly what the operator gave.

A test runs scan, report and trees against such a file name given as an
absolute path, as one starting with //, and as a relative path, and
checks that only that file and its lock file exist afterwards.

Model: opus-5-5

db.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"net/url"
 	"os"
 	"path/filepath"
 	"slices"
@@ -107,7 +108,19 @@ const reportParams = "mode=ro" +
 // openDB opens the SQLite database at path with the connection
 // parameters params. It does not create or verify the schema.
 func openDB(path, params string) (*sql.DB, error) {
-	db, err := sql.Open("sqlite", "file:"+path+"?"+params)
+	// The path is escaped into a file: URI, so ?, # and % in it stay
+	// part of the file name. SQLite reads what follows file:// up to
+	// the next / as a host name, so an absolute path goes after an
+	// empty host (file:///abs) and a relative path goes without one
+	// (file:rel).
+	uri := url.URL{
+		Scheme:   "file",
+		OmitHost: !filepath.IsAbs(path),
+		Path:     path,
+		RawQuery: params,
+	}
+
+	db, err := sql.Open("sqlite", uri.String())
 	if err != nil {
 		return nil, fmt.Errorf("open database %s: %w", path, err)
 	}