install: document parakeet linux path + cross-platform install flow
Rewrites the Parakeet section of INSTALL.md to cover both macOS
(Apple Silicon + Xcode CLT + FluidAudio helper) and Linux x86_64
(NeMo + torch via uv sync --extra parakeet-nemo, ~2.4 GB HF cache).
make install now runs the right prep step per OS, so the manual
instructions are simplified to the config snippet plus one bullet
per platform.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>