v3.25.2 — poll_channel(allow_idle=True) sees channels radiod holds at 0 Hz (#6)

Fixed

• poll_channel() can now see a channel radiod holds at 0 Hz (#6).
  radiod answers a poll for a channel it is still reaping after
  remove_channel() — or one parked at 0 Hz, or one a dynamic-create build
  minted for the poll itself — with the same frequency = 0 reply it gives
  for an unknown SSRC. poll_channel() discarded those as "no such
  channel", so a purging SSRC was indistinguishable from a free one and any
  "is this SSRC free?" logic re-created the dying channel (answers status,
  never emits RTP). New allow_idle=True returns that reply as a
  ChannelInfo with frequency == 0.0, meaning "radiod currently answers
  for this SSRC at 0 Hz — not free to reuse". Default behaviour is
  unchanged; expected_freq still rejects 0 Hz. remove_channel()'s
  docstring now states the purge window.

Commits: 46e770c (fix), 0c75263 (release). Wire behaviour reasoned against HamSCI/ka9q-radio f32654d9; compat pin unchanged (cd44bbdd).