Strengthen homepage with code-backed Solar architecture

[lisihao]

Summary

• Strengthen the README framing so it better reflects the current Solar architecture.
• Add a code-backed architecture map at docs/solar-architecture-code-map.md.
• Surface the implemented actor/host model, logical operator catalog, operator runtime, DAG scheduler, workflow/architecture guards, capability capsules, session log/projection, Deep Research gate, and plugin framework.
• Keep the public tone grounded: stronger architecture claims, but tied to actual repository code surfaces.

Notes

This is documentation-only. No runtime code changes.

Why

The previous homepage described the high-level direction, but underrepresented the system-software work already present in the repo: actor/host registry, logical operator binding, operator runtime leases, capability capsules, event sourcing, and deterministic research gates.