bluez_native 0.3.2

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Add a Sensirion SCD41 CO2 gadget example: example/scd41_co2_sensor.dart, with the wire protocol factored into example/sensirion_gadget.dart. It covers scan, info, live values, history download with sparkline and CSV export, logging interval, LED brightness, rename and forced recalibration.

The gadget protocol is not published as a spec, so the UUIDs, byte layouts and the 21 sample layouts come from Sensirion's own firmware — arduino-ble-gadget for the services, advertisement header and download packets, arduino-upt-core for the sample layouts. Any gadget built on that library works, not only the SCD4x build.

Two paths to live values: scan and monitor decode the manufacturer-specific advertisement data and need no connection at all, while live connects and subscribes, which updates faster.

Verified against a production MyCO2, whose GATT table diverges from the reference firmware more than its shared service UUIDs suggest; the divergences and how each was established are documented in example/README.md.

Library code is unchanged — examples and tests only.

Fix a use-after-free that took down the process. Every C ABI entry point cast the opaque handle straight to a BridgeContext and dereferenced it, so any call arriving after bluez_client_destroy followed freed memory. It presented as a segfault inside __dynamic_cast on a freed sdbus connection, two frames removed from anything the caller wrote and with no diagnostic.

The ordering is not exotic: BlueZGattCharacteristic captures the client handle when it is constructed, so a retained characteristic keeps using it after BlueZClient.close, bypassing the null check on the client's own methods.

Handles are now tokens resolved through a registry, and an entry point whose handle names nothing does nothing. The token is a counter rather than the address of the context: an allocator can reissue a freed address, and a stale handle would then name a different live client and act on it, which is defined behaviour and the wrong client.

Lookup returns a shared_ptr so a client cannot be destroyed while a call is using it. Destruction removes it from the registry immediately, so no later call finds it, while a call already in flight finishes against a connection that is still alive.

Destroying a handle that was never issued, or was already destroyed, is not an error.