PTP: don't report the pre-step offset

After a clock step, status.ptp showed the whole step (~1.79e18 ns) as
offset_ns until the next Sync, and the 60 s summary reported it as
max |offset|. Offset is reset to 0 and the summary restarts on a step.
Verified: status goes 0 -> pull-in values; first summary max 24960 ns.

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

components/aes67_ptp/ptp_clock.c

@@ -268,6 +268,9 @@ static void servo(int64_t offset)
         ESP_LOGI(TAG, "clock stepped by %+lld ns (%s), frequency %+.3f ppm", -offset, esp_err_to_name(err),
                  s.freq_ppb / 1000);
         s.stepped = true;
+        s.offset_ns = 0;    // the pre-step offset is history: not for status or the summary
+        s.sum_max_ns = 0;
+        s.sum_start_us = 0;
         s.prev_t2 = 0;      // rate across the step is meaningless
         s.bad = 0;
         set_locked(false);