feat(ble): replace blocking scan with advertisement observation

The one-shot scan kept only the top six devices by RSSI and dropped the
advertisement payload, so a distant beacon lost its slot to nearby phones
and its data was unreachable -- wrong on every axis for reading sensors
that broadcast in their adverts.

Replace it with a continuous observer: observe(filter)/observed()/
unobserve()/isObserving(). BleObservation carries the raw advertisement
bytes for Lua to parse, and BleFilter keeps only adverts matching a
service-data UUID or manufacturer id.

native/include/lua/providers.h

@@ -246,10 +246,24 @@ public:
   virtual Status forget() = 0;
 };
 
-struct BleDevice {
-  std::string name;
+// One sighting of an advertising device, coalesced to its latest advert. The
+// payload is the raw advertisement bytes; parsing the AD structures (and any
+// beacon format inside them) is the caller's job, so a new sensor format never
+// touches this seam.
+struct BleObservation {
   std::string address;
+  std::string name;
   int32_t rssi;
+  std::string payload;
+  int32_t lastSeenMs;
+};
+
+// Kept adverts must carry one of these service-data UUIDs or one of these
+// manufacturer ids; an empty filter keeps everything. Applied before an advert
+// is buffered, so unwanted devices never occupy a slot.
+struct BleFilter {
+  std::vector<std::string> services;
+  std::vector<int32_t> manufacturers;
 };
 
 class BleProvider {
@@ -258,7 +272,10 @@ public:
   virtual Status init(const std::string* name) = 0;
   virtual void deinit() = 0;
   virtual bool isInitialized() const = 0;
-  virtual Status scan(int32_t durationMs, std::vector<BleDevice>& devices) = 0;
+  virtual Status observe(const BleFilter& filter) = 0;
+  virtual void unobserve() = 0;
+  virtual bool isObserving() const = 0;
+  virtual void observed(std::vector<BleObservation>& out) = 0;
   virtual Status connect(const std::string& address) = 0;
   virtual void disconnect() = 0;
   virtual bool isConnected() const = 0;