Phase 9 analytics (#41)

* Phase 9.1: Initial Analytics with Basic Aggregates and Window Functions

* Session complete: Phase 8 released, Phase 9.1 implemented

* Phase 9.2 Complete: Advanced Aggregate Functions

Implementation: 7 advanced aggregate functions with 49 tests (100% passing)

Functions: STDDEV, VAR, MEDIAN, PERCENTILE, MODE, CORR, COVAR

Algorithms: Welford for stability, linear interpolation for percentiles

Tests: 72 total analytics tests, 100% coverage

Docs: Completion report, release notes, session summary

Phase 9 Progress: 43% complete (3/7 sub-phases done)

* Documentation: Comprehensive update v1.3.5 Phase 9 Analytics Complete

* Cleanup: Remove obsolete documentation files - 24 files deleted for cleaner repository

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Co-authored-by: MPCoreDeveloper <posseth-software@outlook.com>

Author: MPCoreDeveloper