`BroadcastReceiver.receive_nowait()` treated `seq` as a deque index
but subtracted `BroadcastState.maxlen` without counting the first
invalid index. A one-slot queue thus claimed it dropped zero values
after its subscriber missed one.
Include that first displaced value in the count. Preserve the
existing Tokio-style reset to the oldest retained item.
Also, cover exact loss reporting and recovery for one- and
three-slot retention windows.
Prompt-IO: ai/prompt-io/opencode/20260811T233833Z_7cbd64ee_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi