Docs: visual evolution of Stackwich (EVOLUTION.md + deterministic SVG timeline) #48

Add a Git-friendly history of Stackwich from the first release to v3.5.0, with #47 (policy-rev 9, 3.6.0) as next.

Deliverables

• docs/EVOLUTION.md: Mermaid timeline (renders on GitHub), per-version table (date, version, policy-rev, change, value added), themes (token leanness, failure ladder, Codex/Grok ports, install/upgrade).
• docs/assets/stackwich-evolution.svg: hand-written deterministic SVG (no random values or timestamps), referenced from the doc.

Acceptance

• Facts come only from releases/merged PRs.
• Mermaid validates; SVG renders without overlap.
• python3 tools/validate.py passes.
• Work on branch docs/evolution; no push or merge without owner's yes.