docs: add Core 2.0 partner handoff package for v2.0.0-draft.2

[DigiEmu]

Summary

This PR adds a partner-facing technical handoff document for DigiEmu Core 2.0 Draft 2.

The handoff package explains what partners should test, review, and provide feedback on using the current v2.0.0-draft.2 milestone.

Added

• docs/CORE_2_PARTNER_HANDOFF.md

Updated

• docs/SPEC_INDEX_v1.0.md
• docs/CORE_2_STATUS.md
• docs/CORE_2_PARTNER_INTEGRATION_NOTES.md

Handoff Scope

The document covers:

• current milestone: v2.0.0-draft.2
• draft / pre-release status
• local Go test path
• experimental conformance CLI
• JSON conformance report output
• Docker-based conformance path
• OpenAPI contract draft
• expected results
• review questions for partners
• requested feedback
• out-of-scope boundaries

Validation

• go test ./... passes

Compatibility

• Documentation-only change
• No production code changed
• No CLI behavior changed
• No schemas changed
• No testdata changed
• No tag changes
• No v1.0 behavior changed
• Core 2.0 remains draft/pre-release unless explicitly marked stable