cmd/speedtest-sync: fail-fast on bad timestamps + minor cleanups

- Skip rows with null/malformed timestamps instead of silently
  re-stamping with time.Now() (data corruption hidden behind a
  fallback). Drops the time import.
- Replace ad-hoc parentDir() with stdlib filepath.Dir().
- Add t.Fatal error checks to TestExtractRows_SkipsBadExtra
  (CREATE TABLE / INSERT) and switch sql.Open to checked form.
- New TestExtractRows_SkipsBadTimestamp covers the null-ts path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/speedtest-sync/main.go

@@ -6,6 +6,7 @@ import (
 	"flag"
 	"log"
 	"os"
+	"path/filepath"
 	"strconv"
 	"time"
 
@@ -99,17 +100,8 @@ func readCursor(path string) int64 {
 }
 
 func writeCursor(path string, id int64) error {
-	if err := os.MkdirAll(parentDir(path), 0o755); err != nil {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return err
 	}
 	return os.WriteFile(path, []byte(strconv.FormatInt(id, 10)), 0o600)
 }
-
-func parentDir(p string) string {
-	for i := len(p) - 1; i >= 0; i-- {
-		if p[i] == '/' {
-			return p[:i]
-		}
-	}
-	return "."
-}