Document the precomputed binary feature format

[dnvt]

src/precompute.rs defines a binary cache format for precomputed MFCC features. The format works but isn't documented outside the source code.

Goal: Add a section to the README or docs/ explaining:

• The binary format layout (header, data sections, manifest JSON)
• How to generate precomputed features
• How to use them with the training loop
• When precomputing is worth it vs. on-the-fly extraction