docs: cosmic green banner, branded diagram suite & README restructure

[ancongui]

Summary

Brings the PyFly README to the Rust sibling's visual bar, in PyFly's own identity — PyFly is the green firefly of the Firefly family (same firefly-in-the-dark language, recolored to Python green; the logo's green sparkles are the family's fireflies).

Banner & diagrams (assets/)

• banner.svg — dark cosmic banner: the actual logo snake (cropped from pyfly-logo.png) + a Maven Pro vectorized pyFly wordmark + green fireflies and light trails.
• Six branded diagrams in the book palette — architecture, hexagonal, auto-configuration, request-lifecycle, distributed-patterns, ecosystem — with brand icons (Python, Rust, Go, .NET, Spring, Angular, Postgres, Mongo, Redis, Kafka…), measured text (no wrapping/overflow), and an automated zero-overlap check. The ecosystem now includes Rust.

Every SVG is self-contained and GitHub-sanitization-safe (no scripts / external refs / web fonts) and reads correctly in light and dark mode.

README restructure

• Banner + nav bar + table of contents
• Merged The Problem + What is PyFly? → Why PyFly?; elevated Quickstart
• New Architecture section embedding four diagrams (the ASCII hexagon is now a real diagram)
• distributed-patterns + ecosystem diagrams embedded; Rust added to the ecosystem table
• Trimmed the stale inline changelog to a teaser; version badge 26.06.103 → 26.06.113; added Contributing

Book & docs

• book/art/cover.{svg,png} + gen_cover.py — cover title now uses the Maven Pro wordmark (matches the banner); PNG render falls back to resvg
• New CONTRIBUTING.md, assets/README.md (brand kit), and assets/tools/ — self-contained, regenerable build tooling (build_brand_assets.py, Simple Icons vendored)

Verification

• ✅ All 8 SVGs valid XML + sanitization-safe
• ✅ README: 29/29 anchors resolve; all relative/doc links check out
• ✅ Diagrams rendered exact-size (resvg) and reviewed; overlap checker reports none
• The master logo PNG is unchanged; no library code touched