Persist exact frame indices for audio crops

When a crop is cut using floating-point seconds, storing only rounded timestamps can make the saved audio impossible to reproduce byte-for-byte: truncation to integer frames before rounding and truncation after rounding may differ by a sample. Persist the actual start/end frame indices, sample rate, channel, and renderer version at crop creation. For older immutable artifacts, a bounded search for a unique byte-exact source interval can recover crop provenance, but derived bounds do not prove the original in-memory timestamps or the model's generation-time input.

Applies when: audio crops must be replayed exactly from a retained source recording. Keywords: audio crop frame indices rounded seconds sample provenance replay verification