Trim README to pitch + examples, move language reference to MANUAL.md

README was 965 lines mixing sales pitch with full language reference.
Trim to 401 lines: motivation, comparison table, getting started, the
three C side-by-side examples, and loader/session overview. All language
details (types, bindings, control flow, maps, structs, protocols, etc.)
now live in doc/MANUAL.md (renamed from doc/manual.md).

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