feat(v2): add experimental runtime and negotiation

[PsiACE]

Summary

• Generate bindings from the pinned schema-v2.0.0-alpha.3 schema and expose a strict experimental v2 runtime.
• Add active session handling and one-shot v1/v2 negotiation while keeping each version isolated after initialization.

Related issues

None.

Testing

• make check
• make gen-check
• make test (213 passed, 1 skipped)
• make docs-test

Docs & screenshots

Added docs/experimental-v2.md. No screenshots are needed.

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.