Step 6.3: PTP hybrid mode (unicast Delay_Req/Resp), step 6 done

- As TimeReceiver in ptp.mode hybrid: Delay_Req unicast to the GM (IP
  from its Announce, MAC recorded by the RX hook from its Sync frames),
  unicastFlag set.
- As TimeTransmitter: a Delay_Req with the unicastFlag gets a unicast
  Delay_Resp; multicast requests get multicast replies.
- EMAC timestamps every received frame (en_ts4all): its PTP filter left
  unicast Delay_Req unstamped ("no HW RX timestamp").
- A unicast Delay_Resp's logMessageInterval 0x7F is ignored (use
  ptp.log_delay_req); log_us() clamps to -7..6 (the shift was undefined).
- Verified vs ptp4l --hybrid_e2e 1: board hybrid TimeReceiver 18
  Delay_Req in 20 s, all answered, locked; board TimeTransmitter
  (p1 100): tcpdump shows Sync/Follow_Up/Announce to 224.0.1.129,
  Delay_Req 192.168.192.233 -> .244 [unicast] and Delay_Resp .244 ->
  .233 [unicast] within 0.4 ms; ptp4l s2.
- Docs: TimeTransmitter/hybrid notes; step 6 ticked.

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

components/aes67_ptp/ptp_hw.h

@@ -19,6 +19,9 @@ esp_err_t ptp_hw_init(esp_eth_handle_t eth, esp_netif_t *netif);
 // Hardware RX timestamp of an event message (UDP port 319), looked up by type,
 // 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);
+// Same, also returning the sender's Ethernet MAC (for unicast replies as raw frames). src_mac may be NULL.
+bool ptp_hw_rx_ts_mac(uint8_t msg_type, uint16_t seq, const uint8_t *src_port_id, eth_mac_time_t *ts,
+                      uint8_t src_mac[6]);
 esp_err_t ptp_hw_get_time(eth_mac_time_t *t);
 // Frequency correction relative to the nominal rate, in ppb (positive = faster).
 esp_err_t ptp_hw_adj_freq(double ppb);