Function AssembleOpusRecordingPcm

Lays a session's shards out on one 48 kHz timeline and decodes them with one decoder.

• A present shard is decoded packet by packet; exactly SampleCount samples are kept (decoded audio past it is dropped, a shortfall stays silent). Every packet is still decoded so the decoder state stays continuous into the next shard.
• A run of missing indexes becomes one silent gap from the end of the previous shard to the start of the next present one (a trailing run: to the end of the recording), listing the missing indexes.
• A present shard that starts after the end of the previous one leaves a silent gap with no indexes.
• A shard with a packet libopus cannot decode is silenced whole, listed as a gap with its index and in DecodeFailures.
• After any gap the decoder is reset.

The samples are decoded straight into the returned WAV's data section (see allocatePcm16Wav).

The shard count is expectedShards, or the highest present index + 1 when no terminal signal has arrived. The length is capturedSamples, or the end of the last shard when unknown (and never less than the end of the last shard: audio past a stale terminal is kept, not cut).