Docs: Add Phase 121 documentation and update CLAUDE.md #116

Job	Run time
Code Quality Checks	1m 17s
Build & Test (clang-15, Debug)	28s
Build & Test (clang-15, Release)	34s
Build & Test (gcc-13, Release)	50s
Build & Test (gcc-13, Debug)	49s
Performance Regression Check	1m 22s
Sanitizer Tests (ASAN + UBSAN)	34s
Build Summary	3s
	5m 57s

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