Refresh README to cover Phase 9-17 additions

Bring README in line with the current codebase: expand the quantization
section with Phase 10-13 composition ops, norms, recurrent cells, and
attention/FFT; add new sections for the mixed-precision bridges (qbridge
+ fp16/bf16), SIMD performance backend, and importer tooling; extend
the platform feature gates table with FP16, INT16_ACCUM, the twelve
SIMD_* gates, and OPENMP; bump the embedded matrix from five to eight
corners; refresh build examples, project structure tree, and stale
line counts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: danmcleran