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/Kconfig.projbuild

@@ -39,4 +39,18 @@ menu "CSI Node Configuration"
         help
             WiFi channel to listen on for CSI data.
 
+    config CSI_FILTER_MAC
+        string "CSI source MAC filter (AA:BB:CC:DD:EE:FF or empty)"
+        default ""
+        help
+            When set to a valid MAC address (e.g. "AA:BB:CC:DD:EE:FF"),
+            only CSI frames from that transmitter are processed. All
+            other frames are silently dropped. This prevents signal
+            mixing in multi-AP environments.
+
+            Leave empty to accept CSI from all transmitters.
+
+            Can be overridden at runtime via NVS key "filter_mac"
+            (6-byte blob) without reflashing.
+
 endmenu