feat(c6): ESP-NOW cross-node sync — D1 workaround for broken 15.4 RX

After 5 systematic experiments confirmed the 802.15.4 RX path is
unfixable from user code in this IDF v5.4 + C6 combination (D1), add a
parallel sync transport over ESP-NOW. Same TS_BEACON protocol, same
public API (c6_sync_espnow_get_epoch_us / is_valid / is_leader), but
runs on the WiFi MAC layer that ESP-IDF fully supports across every
ESP32 family.

The 802.15.4 code stays in source for when the IDF driver is fixed.
ESP-NOW is the working primary today.

Empirical (single-board COM9 — other 3 boards dropped off USB during
the experiment):
- c6_sync_espnow_init() succeeds: "init done local_id=… leader=
  yes(candidate) period=100ms"
- TX path 100% reliable: tx#101 fail=0 over ~15s at 100ms cadence
- RX awaiting cross-board test once USB-enumeration is restored

Trade vs. 802.15.4 design:
- Loses: "frees WiFi airtime for CSI" property
- Gains: known-working RX path, cross-target (S3 and C6 both)
- Same API surface — consumers swap transports without code change

Files:
- main/c6_sync_espnow.{h,c} — new module, ~210 lines
- main/CMakeLists.txt        — add to SRCS (always built, used on any target)
- main/main.c                — init after WiFi STA up, skip on QEMU mock
- test/capture-3board-experiment.py — surface c6_espnow log lines
- docs/WITNESS-LOG-110.md    — new §D-workaround documenting the pivot

Ref: ruvnet/RuView#762 / D1 known-issue / draft PR #764

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

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

@@ -36,6 +36,7 @@
 #include "c6_twt.h"                /* ADR-110: TWT (no-op stub on S3) */
 #include "c6_timesync.h"           /* ADR-110: 802.15.4 mesh time-sync (no-op on S3) */
 #include "c6_lp_core.h"            /* ADR-110: LP-core hibernation (no-op on S3) */
+#include "c6_sync_espnow.h"        /* ADR-110 D1 workaround: ESP-NOW sync */
 #ifdef CONFIG_CSI_MOCK_ENABLED
 #include "mock_csi.h"
 #endif
@@ -254,6 +255,18 @@ void app_main(void)
     c6_twt_setup_default();
 #endif
 
+    /* ADR-110 D1 workaround: ESP-NOW cross-node sync. Initialized after
+     * WiFi STA connects (ESP-NOW needs the WiFi driver up). Works on
+     * both S3 and C6 — replaces the broken 802.15.4 RX path in c6_timesync.
+     * Skip on QEMU mock (no real WiFi → no ESP-NOW). */
+#ifndef CONFIG_CSI_MOCK_SKIP_WIFI_CONNECT
+    esp_err_t espnow_ret = c6_sync_espnow_init();
+    if (espnow_ret != ESP_OK) {
+        ESP_LOGW(TAG, "c6_sync_espnow_init failed: %s (continuing without ESP-NOW sync)",
+                 esp_err_to_name(espnow_ret));
+    }
+#endif
+
     /* ADR-039: Initialize edge processing pipeline. */
     edge_config_t edge_cfg = {
         .tier              = g_nvs_config.edge_tier,