Restore common versioned envelope for validate count JSON

[Widthdom]

Summary

• route validate --format count through the shared versioned count envelope while retaining the legacy total_estimated field and pre-limit count semantics
• expose query scope, filters, freshness, validation-data readiness, index completion, and authoritative/degraded state, including unavailable legacy severity filters
• document the contract in both English and Japanese and add the required changelog fragment

Root cause

The validate count fast path called the legacy formatted-count writer directly, bypassing the canonical count-envelope builder and validate-specific readiness metadata.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• focused Issue Restore the common versioned envelope for validate count JSON #4908 tests on net8.0 and net9.0 (4/4 passed on each target after the final origin/main merge)
• full net8.0 Release suite: 11,029 passed, 7 skipped, 1 unrelated load-sensitive failure; the failing Unicode worker test passed in isolation
• full net9.0 Release suite: 10,555 passed, 420 skipped, 1 unrelated timeout; the failing parallel batch-refresh test passed in isolation
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --no-build -c Release --project tools/CodeIndex.Changelog -- check
• Codex adversarial review, including a second round after addressing the legacy severity-schema finding: no remaining correctness, security, or regression issues

Fixes #4908