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/nvs_config.h

@@ -35,6 +35,10 @@ typedef struct {
     uint32_t dwell_ms;                        /**< Dwell time per channel in ms. */
     uint8_t  tdm_slot_index;                  /**< This node's TDM slot index (0-based). */
     uint8_t  tdm_node_count;                  /**< Total nodes in the TDM schedule. */
+
+    /* MAC address filter for CSI source selection (Issue #98) */
+    uint8_t  filter_mac[6];                   /**< Transmitter MAC to accept (all zeros = no filter). */
+    uint8_t  filter_mac_enabled;              /**< 1 = filter active, 0 = accept all. */
 } nvs_config_t;
 
 /**