docs: compact CLAUDE.md, remove redundancy

Merge the duplicated testing/random-data sections and the split Javadoc rules,
fold the per-side encoding/extension checklists and pluggability prose into tighter
form, and trim restated prose. Every rule, command, and the perf code examples
(arena allocation, hot-loop branch-split) are preserved. 429 → 231 lines.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1