docs: add architecture-and-status overview with diagrams

Single contributor-facing document that consolidates parser, engine,
distributed, and transaction status: per-subsystem matrix (done /
partial / in-flight / planned / out-of-scope), 12 mermaid diagrams,
working-tree change inventory grouped by issue, and the roadmap.
Supersedes docs/current-state.md as the "where is the project today"
entry point.

Author: renecannao