chore: add Core 2.0 Draft 4 self-audit script

[DigiEmu]

Summary

This PR adds a local PowerShell self-audit script for the Core 2.0 Draft 4 / post-tag state.

The script verifies the current Draft 4 quality gates, including tag presence, clean working tree, Go tests, conformance CLI output, JSON report fields, basic report fixture consistency, MISSING_REFERENCE schema guardrails, Draft 4 post-tag note presence, and the v1.0.0 stable/latest guardrail.

Added

• scripts/audit_core2_draft4.ps1

Updated

• docs/SPEC_INDEX_v1.0.md
• docs/CORE_2_STATUS.md
• docs/CORE_2_DRAFT_4_READINESS.md
• docs/CORE_2_DRAFT_4_POST_TAG_NOTE.md
• docs/CORE_2_MISSING_REFERENCE_SCHEMA_CONSIDERATION.md

Audit Command

powershell -ExecutionPolicy Bypass -File scripts/audit_core2_draft4.ps1

Current Audit Result

PASS: all Core 2.0 Draft 4 audit checks passed

Checks Covered

• Git working tree is clean
• v2.0.0-draft.4 tag exists
• go test ./... passes
• Experimental conformance CLI reports total=11 passed=11 failed=0
• Experimental conformance CLI --json reports:
  • status=PASS
  • total=11
  • passed=11
  • failed=0
  • report_version=core-2-conformance-report-v1
• Basic conformance report fixture is consistent with 11 cases
• MISSING_REFERENCE is not in the Verify Result v2 schema
• MISSING_REFERENCE schema consideration doc exists
• Draft 4 post-tag note exists
• Docs preserve the v1.0.0 stable/latest guardrail

Compatibility

• No production code changed
• No schemas changed
• No official conformance testdata changed
• No CI changed
• No CLI behavior changed
• No tags changed
• No v1.0 behavior changed