feat(voices): canonical voice corpus + dots.tts-optimized refs

Engine-agnostic voice corpus: canonical source clip + transcript per voice,
per-engine reference sets derived by derive.py from engines.yaml profiles.
First residents donut/glados/emmie/miranda optimized + verified clean for
dots.tts (sentence-bounded ref + accurate transcript — dots leaks reference
audio into output otherwise). canonical/ + transcripts/ tracked; derived/
gitignored (regenerable). Records the dots.tts burn-in in persistent-memory.

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