Improve first-time mdbase onboarding

[callumalpass]

What changed

• preserve the six-scene homepage particle narrative while adding early actions, a concrete interoperability example, an ecosystem map, release status, visitor paths, and fit guidance
• reorganize global and SDK navigation around visitor intent
• add a runnable, zero-account SDK sandbox example
• split private-beta Connect authorization into a complete browser application guide with prerequisites, a provisionable contract, route wiring, and expected results
• clarify the Connect request path, trust vocabulary, and current availability

Why

A first-time visitor could understand the core mdbase idea but had difficulty relating mdbase, Connect, the SDK, implementations, and Runtime or reaching a successful first use. This gives evaluators and developers explicit paths from orientation to a working result.

Validation

• pnpm test
• generated link check across 24 HTML files
• all new local routes return HTTP 200
• downloadable sandbox example executed successfully against the actual mdbase-connect workspace