fix(csi): ADR-018 byte 19 bit 4 now signals ESP-NOW sync too (not just broken 15.4)

WITNESS-LOG-110 prior state had byte 19 bit 4 (cross-node sync valid)
only being set from c6_timesync_is_valid() — but c6_timesync is the
802.15.4 path that D1 documented as unfixable in IDF v5.4 (rx=0 across
every soak we've run). The working transport is c6_sync_espnow (§A0.7,
§A0.10: 99.43-99.56% RX cross-board, 104 µs smoothed-offset stdev),
yet frames from sync'd nodes had bit 4 cleared because the ESP-NOW
path didn't OR into the flag.

Fix: also set bit 4 when c6_sync_espnow_is_valid() — the OR semantic
means a node signals sync from whichever transport is healthy. Host
sees bit 4 set, knows to pair the frame against the most recent sync
packet (§A0.12) from this node_id.

Side effect: this also enables S3 boards to set bit 4 (c6_sync_espnow
works on both targets, c6_timesync is C6-only). So a multi-target
mesh of S3+C6 boards now correctly signals cross-node alignment
regardless of which chips are in the fleet.

Build evidence: C6 image 1019 KB (+16 bytes for the new check),
45% slack unchanged.

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/main/csi_collector.c

@@ -217,9 +217,16 @@ size_t csi_serialize_frame(const wifi_csi_info_t *info, uint8_t *buf, size_t buf
     if (info->rx_ctrl.cwb) flags |= 0x1;            /* bw 40 MHz */
     if (info->rx_ctrl.stbc) flags |= (1 << 2);      /* STBC */
 #endif  /* CONFIG_SOC_WIFI_HE_SUPPORT */
+    /* ADR-018 byte 19 bit 4 = "cross-node sync valid". Two transports can
+     * set it: the original 802.15.4 c6_timesync (broken in IDF v5.4 — D1)
+     * and the ESP-NOW workaround c6_sync_espnow (measured working in §A0.7-
+     * §A0.10). OR them together so frames signal sync from whichever
+     * transport is alive on this node. Host can pair against the sync
+     * packet (§A0.12) once it sees this bit. */
 #if defined(CONFIG_IDF_TARGET_ESP32C6) && defined(CONFIG_C6_TIMESYNC_ENABLE)
     if (c6_timesync_is_valid()) flags |= (1 << 4);  /* 15.4 sync valid */
 #endif
+    if (c6_sync_espnow_is_valid()) flags |= (1 << 4);  /* ESP-NOW sync valid (D1 workaround) */
     buf[18] = ppdu_type;
     buf[19] = flags;
 #else