Adversarial review F9 (Low, cleared as bug): RFC 7587 fixes the SDP at
`opus/48000/2` regardless of internal rate; libopus self-describes
internal rate, so the 24 kHz mono decoder is correct. A 40/60 ms
frame would yield OPUS_BUFFER_TOO_SMALL and rutster's `Option`-shaped
`decode` wrapper routes it to `None` → drop+observe per §3.8 —
already the documented correct behavior. Adds the explicit 20-ms
assumption as a comment so the boundary is visible to future readers.
8a3c9b08c7