feat: BLE passive scanner with RSSI bucketing and MAC hashing

Add passive BLE scan module using NimBLE for WiFi coexistence. Tracks
unique devices per hour with SHA256-hashed MACs, RSSI bucketing
(near/mid/far), max concurrent count, and thread-safe collect/reset.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/src/ble_scanner.h

@@ -0,0 +1,25 @@
+// firmware/src/ble_scanner.h
+#pragma once
+#include <Arduino.h>
+#include <vector>
+
+struct BLEHourlyRecord {
+    uint32_t period_start;
+    uint32_t period_end;
+    int unique_devices;
+    int max_concurrent;
+    int near_count;    // RSSI > -65 dBm
+    int mid_count;     // RSSI -65 to -80 dBm
+    int far_count;     // RSSI < -80 dBm
+    std::vector<String> device_hashes; // SHA256(MAC) first 16 hex chars
+};
+
+// Start continuous passive BLE scan (call once at boot).
+void ble_scanner_start();
+
+// Pause scan for ~3s during HTTP upload.
+void ble_scanner_pause();
+void ble_scanner_resume();
+
+// Collect current hour's record and reset accumulators. Thread-safe.
+BLEHourlyRecord ble_scanner_collect(uint32_t period_start, uint32_t period_end);