Step 3.2: TimeReceiver measurement (GM selection, Delay_Req/Resp, path delay)

- ptp_clock.c replaces the 3.1 logger: UDP/IPv4 multicast, E2E.
  Announce: IEEE 1588 dataset comparison picks the best GM, dropped after
  announceReceiptTimeout x its announce interval. Sync/Follow_Up (two-step
  and one-step) with HW t2 and correctionField. Delay_Req sent as a raw
  frame (DSCP ptp.dscp, TTL 1, clockIdentity = EUI-64 from MAC) with HW
  TX timestamp t3, randomised at the GM's Delay_Resp interval; Delay_Resp
  matched on requestingPortIdentity + seq.
- Path delay corrected for offset drift between t2 and t3 (rate from
  consecutive Syncs) so it is right before the clock is syntonised.
- ptp_hw: ptp_hw_send_event() builds Eth/IPv4/UDP 319 and returns the HW
  TX timestamp.
- Verified vs ptp4l (i210 GM, HP 2530 non-PTP switch, PC 1G / board 100M):
  GM selected, path delay settles at ~10.3 us and stays flat, rate
  -39.8 +-0.4 ppm, offset drifts at -40 us/s (no servo yet).

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

components/aes67_ptp/ptp_hw.h

@@ -20,3 +20,7 @@ esp_err_t ptp_hw_init(esp_eth_handle_t eth, esp_netif_t *netif);
 // sequence ID and sourcePortIdentity (10 bytes). Each record is returned once.
 bool ptp_hw_rx_ts(uint8_t msg_type, uint16_t seq, const uint8_t *src_port_id, eth_mac_time_t *ts);
 esp_err_t ptp_hw_get_time(eth_mac_time_t *t);
+// Send a PTP event message as a raw Ethernet/IPv4/UDP frame (port 319 -> 319) and return its
+// hardware TX timestamp. dst_ip/dst_mac: 224.0.1.129 / 01:00:5e:00:01:81 or the GM unicast.
+esp_err_t ptp_hw_send_event(const uint8_t dst_mac[6], uint32_t dst_ip, uint8_t dscp,
+                            const uint8_t *msg, size_t len, eth_mac_time_t *tx_ts);