docs: overhaul README with honest assessment and H100 metrics

Rewrites README with:
- Current H100 benchmark numbers (8,698x vs traditional, 3,005x vs CUDA Graphs)
- Honest assessment section: what works, what's partial, what's missing
- GPU actor lifecycle documentation with code examples
- Updated architecture diagram showing supervisor + actor pool
- Accurate crate status table (Stable/Scaffold/Partial)
- Updated test count (1,496)
- Roadmap: completed items and next priorities
- Links to academic proof, gap analysis, methodology docs

Removes outdated RTX Ada numbers and inflated claims.
Adds honest qualifications throughout.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Ubuntu