Complete Section 6 specification with all reviewer feedback

Major additions:
- MEM-3: Zero-copy accumulator algorithm
- DH-2: Formula-based float binning
- API-1: binning_formulas parameter
- FR-9: Error handling with PerformanceWarning

Reviewers: Gemini (approved), GPT (approved)
Lines: 1855 (+396, +27%)
Status: Production-ready

Author: miranov25