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/test/fake_providers.h

@@ -276,9 +276,10 @@ struct Wifi : WifiProvider {
 };
 
 struct Ble : BleProvider {
-  int32_t duration = 0;
   std::string value;
   bool initialized = false;
+  bool observing = false;
+  BleFilter filter;
 
   Status init(const std::string*) override {
     initialized = true;
@@ -286,12 +287,17 @@ struct Ble : BleProvider {
   }
   void deinit() override { initialized = false; }
   bool isInitialized() const override { return initialized; }
-  Status scan(int32_t durationMs, std::vector<BleDevice>& devices) override {
-    duration = durationMs;
-    const BleDevice device = {"tag", "aa:bb", -60};
-    devices.push_back(device);
+  Status observe(const BleFilter& next) override {
+    filter = next;
+    observing = true;
     return Status::success();
   }
+  void unobserve() override { observing = false; }
+  bool isObserving() const override { return observing; }
+  void observed(std::vector<BleObservation>& out) override {
+    const BleObservation device = {"aa:bb", "tag", -60, "payload", 1234};
+    out.push_back(device);
+  }
   Status connect(const std::string&) override { return Status::success(); }
   void disconnect() override {}
   bool isConnected() const override { return true; }

native/test/runtime_test.cpp

@@ -121,10 +121,14 @@ int main() {
   run(state, "assert(not ble.isInitialized())\n"
              "assert(ble.init())\n"
              "assert(ble.isInitialized())\n"
-             "assert(ble.scan()[1].address == 'aa:bb')\n"
+             "assert(ble.observe({ services = { '181A' } }))\n"
+             "assert(ble.isObserving())\n"
+             "assert(ble.observed()[1].address == 'aa:bb')\n"
+             "assert(ble.observed()[1].payload == 'payload')\n"
              "assert(#ble.read('svc', 'chr') == 3)\n"
              "assert(ble.write('svc', 'chr', 'x\\0y'))");
-  assert(bench.ble.duration == 3000);
+  assert(bench.ble.filter.services.size() == 1);
+  assert(bench.ble.filter.services[0] == "181A");
   assert(bench.ble.value.size() == 3);
 
   run(state, "fired = 0\n"