feat: add MAC address filter for ESP32 CSI collection

In multi-AP environments, CSI frames from different access points get
mixed together, corrupting the sensing signal. Add transmitter MAC
filtering so only frames from a specified AP are processed.

Implementation:
- csi_collector: filter in wifi_csi_callback by comparing info->mac
  against configured MAC; log transmitter MAC in periodic debug output
- csi_collector_set_filter_mac(): runtime API to enable/disable filter
- Kconfig: CSI_FILTER_MAC option (format "AA:BB:CC:DD:EE:FF")
- NVS: "filter_mac" 6-byte blob overrides Kconfig at runtime
- nvs_config: parse Kconfig MAC string at boot, load NVS override
- main: apply filter from config after csi_collector_init()

When no filter is configured (default), behavior is unchanged —
all transmitter MACs are accepted for backward compatibility.

Fixes #98

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

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

@@ -22,12 +22,28 @@
 /** Maximum number of channels in the hop table (ADR-029). */
 #define CSI_HOP_CHANNELS_MAX 6
 
+/** Length of a MAC address in bytes. */
+#define CSI_MAC_LEN 6
+
 /**
  * Initialize CSI collection.
  * Registers the WiFi CSI callback.
  */
 void csi_collector_init(void);
 
+/**
+ * Set a MAC address filter for CSI collection.
+ *
+ * When set, only CSI frames from the specified transmitter MAC are processed;
+ * all others are silently dropped. This prevents signal mixing in multi-AP
+ * environments.
+ *
+ * Pass NULL to disable filtering (accept CSI from all transmitters).
+ *
+ * @param mac  6-byte MAC address to accept, or NULL to disable filtering.
+ */
+void csi_collector_set_filter_mac(const uint8_t *mac);
+
 /**
  * Serialize CSI data into ADR-018 binary frame format.
  *