Harden production ceremony and release validation

[mellowcroc]

Summary

• update the production Go toolchain and release validation to Go 1.26.6
• enforce the reviewed destination-v2 circuit identity in production ceremony-definition validation, including authenticated definition loads
• retain the early production init check so an invalid circuit fails before large Phase 1 artifacts are generated
• remove the operator-controlled Windows signed-release claim; the current workflow can only publish explicitly unsigned previews
• pin Windows release workflow actions, restrict write permission to the publishing job, and validate dispatch tags without shell interpolation
• update the Windows release runbook and add regression coverage

Validation

• go test ./internal/mpcceremony ./cmd/mpc-ceremony ./scripts/verify-mpc-build-metadata
• complete Go package suite, with memory-heavy circuit packages rerun individually
• go vet ./...
• desktop Vitest suite: 14 tests passed
• actionlint and YAML validation for changed workflows
• govulncheck ./...: no reachable vulnerabilities
• Relay/mpc-ceremony ceremony-kit binary compatibility rehearsal