docs(stt): add whisper.cpp DTW word-timestamp POC plan

Standalone implementation plan (outside the OpenScreen repo) for a coding
agent to run a fair, precision-matched comparison of whisper.cpp's native
DTW word-timestamps (WHISPER_AHEADS_SMALL, libwhisper linked directly) against
the current CTranslate2 baseline, plus Vulkan/CUDA speed numbers. This is the
test the prior whisper.cpp evaluation never ran correctly (it hit the
non-DTW heuristic path and a q5_1 model with no alignment-head data).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: EtienneLescot