ADR-110: ESP32-C6 firmware extension (#764)

Closes the firmware-side ADR-110 design at v0.7.0-esp32 after a 38-iter /loop SOTA sprint.

Headline (bench, COM9+COM12 ESP32-C6):
- 99.56% cross-board RX, 104.1 µs smoothed offset stdev (≤100 µs §2.4 target met)
- 3.95× EMA suppression, 1.4 ppm crystal skew preserved

4 firmware releases: v0.6.7 / v0.6.8 / v0.6.9 / v0.7.0-esp32.
42 ADR-110 unit tests, 1761 v2 workspace tests, full Firmware CI + QEMU green.

firmware/esp32-csi-node/main/c6_softap_he.h

@@ -0,0 +1,66 @@
+/**
+ * @file c6_softap_he.h
+ * @brief ESP32-C6 soft-AP with Wi-Fi 6 (HE) capability + TWT Responder.
+ *
+ * ADR-110 §B1/B2 cheap-unblock: turn one C6 board into the iTWT-capable
+ * AP that the C6-DevKit-on-the-shelf-only bench is missing. A second C6
+ * board in STA mode can then negotiate a real iTWT agreement against
+ * this AP and measure deterministic CSI cadence — without buying an
+ * 11ax router.
+ *
+ * Build-gated by CONFIG_C6_SOFTAP_HE_ENABLE (default n). When disabled,
+ * all functions become no-ops so non-AP firmwares pay zero overhead.
+ *
+ * NVS overrides (read at boot if present, fall back to Kconfig defaults):
+ *   softap_ssid   (string, up to 32 chars)
+ *   softap_psk    (string, 8..63 chars)
+ *   softap_chan   (u8, 1..13)
+ */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#if defined(CONFIG_IDF_TARGET_ESP32C6) && defined(CONFIG_C6_SOFTAP_HE_ENABLE)
+
+/**
+ * Bring up the soft-AP in AP+STA mode with HE (Wi-Fi 6) advertised and
+ * TWT Responder=1 if the IDF build supports it. Idempotent — safe to
+ * call once during boot after `esp_wifi_init()`. Returns the channel
+ * the AP is actually running on (may differ from Kconfig if the IDF
+ * scanner picks a clearer channel).
+ */
+esp_err_t c6_softap_he_start(uint8_t *out_channel);
+
+/**
+ * True after the IDF reports the AP has started successfully.
+ */
+bool c6_softap_he_is_up(void);
+
+/**
+ * Number of currently associated stations (read-only, refreshed on the
+ * WIFI_EVENT_AP_STACONNECTED/DISCONNECTED events).
+ */
+uint8_t c6_softap_he_sta_count(void);
+
+#else  /* disabled — no-op stubs */
+
+static inline esp_err_t c6_softap_he_start(uint8_t *out_channel)
+{
+    if (out_channel) *out_channel = 0;
+    return ESP_OK;
+}
+static inline bool    c6_softap_he_is_up(void)     { return false; }
+static inline uint8_t c6_softap_he_sta_count(void) { return 0; }
+
+#endif
+
+#ifdef __cplusplus
+}
+#endif