Split framework-agnostic core from Express adapter

[mmurrs]

Context

dual402 is currently packaged as an Express-first middleware. The source is now split into focused modules, but core payment behavior still assumes Express request/response objects and mppx/express.

Now that the public-release hardening and docs/release polish are done, the next architectural step is a framework-agnostic core with thin adapters.

Goal

Expose reusable core primitives for x402 + MPP charge/discovery behavior, then keep Express as the first adapter.

Proposed Shape

• Move protocol-neutral logic into a core layer:
  • config validation and resolved config
  • payment requirement/challenge construction
  • x402 verify/settle orchestration
  • schema/discovery metadata construction
  • shared paid-route model
• Keep Express-specific behavior in ./express:
  • Express middleware signatures
  • res.status(402) interception
  • request URL/base URL extraction
  • mppx/express integration
• Preserve existing dual402 and dual402/express imports.
• Add a deliberate dual402/core export only when the core API is ready.

Acceptance Criteria

• Existing Express API remains backward compatible.
• Existing tests still cover Express behavior.
• Core code can be exercised without importing Express types.
• Adapter boundary is documented in ARCHITECTURE.md.
• README continues to present Express as the default path, with core/adapters framed as extension points.