Fix notification subscription state reporting

[jwinarske]

BlueZGattCharacteristic.notifying reported the wrong value while notifications were arriving. Two independent bugs produced it, and neither is visible to a test that only checks whether bytes show up.

Notifying and MTU were never forwarded

The characteristic PropertiesChanged handler returned early unless the update carried Value, so notifying kept its discovery-time value for the lifetime of the characteristic.

BlueZGattCharProps gains a changedMask field and a GattCharChangedBit enum. The field is appended last in glz::meta and read last in codec.dart, so the layout of the preceding fields is unchanged. Those two must be edited together — a mismatch is silent corruption, not a compile error. The frozen vectors in codec_test.dart caught the layout change immediately, which is what they exist for.

0x03 now routes through BlueZGattCharacteristic.mergeChanged, which updates cached properties and emits on the value stream only when the update actually carried a value. Without that check a Notifying-only change would push an empty payload to listeners.

The subscription was registered too late

subscribe_char_notify ran inside the StartNotify reply handler, but BlueZ
emits Notifying=true while servicing the call. The listener was attached after the signal had already been delivered. Fixing the first bug alone changed
nothing, which is how this one surfaced.

The subscription is now registered before the call and rolled back if StartNotify fails. On the stop path the unsubscribe is deferred until the property read resolves, so a Notifying=false arriving after the reply is still observed.

Post-condition check on Start/StopNotify

A successful method reply does not prove the notification state changed. BlueZ
scopes a notification session to the D-Bus client that requested it, so a client that does not outlive the call sees StartNotify succeed while Notifyingstays false and noPropertiesChanged` ever arrives. That presents as the peripheral having stopped responding.

The property is now read back after the reply, asynchronously — the reply handler runs on the event loop thread, where a synchronous D-Bus call can deadlock. If the read itself fails it reports success: an unreadable property is not evidence the operation failed, and failing there would break working callers.

Note the read must use the live property. Checking the cached value would fail
every healthy subscription, since the cache is exactly what was broken.

Other changes

Variant extraction in the signal handler is guarded. Variant::get throws when a peer sends an unexpected type, and an exception escaping the handler takes
down the D-Bus event loop thread along with every other subscription on it.

glaze_meta.h records that its length and count prefixes are uint32 while other implementations of the same encoding use uint64. Reading a uint64 prefix
consumes the first four bytes of string data as the high half of the length, producing a value far from anything recognizable.

Descriptions in the README, pubspec, changelog, library doc comment, and two headers now describe the delivery mechanism directly rather than naming an internal channel taxonomy.

Testing

• 42/42 native tests, 16/16 Dart tests, dart analyze clean
• Each commit builds and passes independently
• clang-tidy and clang-format applied. The reply-handler lambda keeps by-value
  parameters with a NOLINT: sdbus::apply invokes handlers by value, so
  reference parameters do not bind and the template fails to instantiate
• Verified against a GoPro MAX2 over BLE using
  example/notify_regression.dart, which asserts the notifying transitions
  rather than only that bytes arrive:

  [pass] notifying is false before subscribing
  [pass] startNotify() succeeds on a healthy link
  [pass] notifying is true after subscribing
  notification: 08 13 00 08 01 00 0a 01 00
  [pass] notification received with a non-empty payload
  [pass] stopNotify() succeeds
  [pass] notifying is false after unsubscribing

Against the pre-fix build, notifying is true after subscribing fails while
every other check passes.

[jwinarske]

cc @jaydon2020