fix(c6): TWT INVALID_ARG graceful + ch26 + diagnostic counters (ADR-110 D1)

After 3 systematic hypotheses tested + rejected (radio coex, OpenThread
shadowing, manual RX re-arm), the 802.15.4 leader-election bug is
narrowed to: TX path works perfectly (~10/s clean, 0 fail), but the RX
path stops after exactly 1 frame. Manual esp_ieee802154_receive() from
either callback bootloops the driver (verified across all 3 boards).

The IDF reference example uses the same handle_done-only pattern as
this code, implying the driver should auto-restart RX — but empirically
doesn't here. Either a half-duplex radio state issue or an IDF v5.4
bug. Tracked as known issue D1 in WITNESS-LOG-110.

Changes shipped:
- c6_twt.c: ESP_ERR_INVALID_ARG added to graceful-fallback list
  (empirically: ruv.net AP advertises TWT Responder=0, IDF driver
  validates against AP HE capability and rejects with INVALID_ARG)
- c6_timesync.c: diagnostic counters (s_tx_count, s_tx_fail, s_rx_count,
  s_rx_magic_match) + per-10-beacon log line preserved so future
  investigation has the diagnostic harness ready
- sdkconfig.defaults.esp32c6: 15.4 channel default 15 → 26 (non-overlap
  with WiFi 2.4 GHz channels), OpenThread disabled (we use raw 15.4)
- promiscuous=true on the radio (broadcast frames addressed to 0xFFFF)
- WITNESS-LOG-110 §D1 expanded with the full diagnostic trace +
  3-hypothesis investigation record

Cross-node sync claim (B3) BLOCKED until either an IDF maintainer
trace or a working multi-board reference is available. The other
three SOTA dimensions (HE-LTF, TWT cadence, 5 µA hibernation) are
also still unverified and need different hardware (11ax AP, INA meter)
— honestly recorded in §B.

Tracking: ruvnet/RuView#762, task #30 closed as known-issue.

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

firmware/esp32-csi-node/sdkconfig.defaults.esp32c6

@@ -28,17 +28,22 @@ CONFIG_ESP_WIFI_CSI_ENABLED=y
 # on chips that have HE support (C6/C5). WPA3 is opt-in:
 CONFIG_ESP_WIFI_ENABLE_WPA3_SAE=y
 
-# ── ADR-110 P4: 802.15.4 + OpenThread (MTD) ──
-# IEEE 802.15.4 PHY + OpenThread Minimal Thread Device for mesh time-sync.
-# MTD is lighter than FTD (no router/leader code) — perfect for sensor nodes.
+# ── ADR-110 P4: 802.15.4 (raw, no OpenThread) ──
+# IEEE 802.15.4 PHY enabled for our raw beacon protocol in c6_timesync.c.
+# OpenThread is DISABLED — empirically (ch15 + ch26 tested with the same
+# negative result), enabling OpenThread MTD caused our weak-symbol overrides
+# of esp_ieee802154_receive_done/transmit_done to never fire, breaking
+# leader election. Raw 802.15.4 mode is what we actually need: a private
+# mesh protocol on a private channel, no Thread network attach.
 CONFIG_IEEE802154_ENABLED=y
-CONFIG_OPENTHREAD_ENABLED=y
-CONFIG_OPENTHREAD_MTD=y
-CONFIG_OPENTHREAD_FTD=n
-CONFIG_OPENTHREAD_RADIO=n
-# Disable joiner / commissioner — we use a pre-shared network key in NVS.
-CONFIG_OPENTHREAD_JOINER=n
-CONFIG_OPENTHREAD_COMMISSIONER=n
+CONFIG_OPENTHREAD_ENABLED=n
+
+# ADR-110 P4: 802.15.4 channel override.
+# Default Kconfig value is 15 (2425 MHz). On the 2.4 GHz radio that's
+# directly under WiFi channel 5 (2432 MHz). Channel 26 = 2480 MHz is on
+# the WiFi guard band above channel 14, giving the 15.4 path room to RX
+# without competing with WiFi traffic for radio time.
+CONFIG_C6_TIMESYNC_CHANNEL=26
 
 # ── ADR-110 P5: LP-core (deep-sleep coprocessor) ──
 # Enable the LP RISC-V core so c6_lp_core.c can ship a wake-on-motion stub.