feat(observability): export readiness evidence gauges

[seonghobae]

Scope

This Draft preserves only the readiness-metrics feature: expose enterprise-sale/readiness gauges through /metrics, reuse existing KPI/commercial-readiness authority, document their semantics, and keep HTTP regression coverage. It does not create a second readiness authority.

Protected prerequisite repair — 2026-09-05 KST

The former base #153@b663f9d200e5f385c7dd067d074940a02836c68e is superseded. Its valid workflow/runner foundation is now protected truth through #159 at main@5829a0f08d78de464dd24393ce5d0f25fba9d126.

Fresh comparison of the old feature head against #153 proves the unique readiness delta is exactly four non-workflow files: CHANGELOG.md, docs/analytics/soc-kpis.md, docs/commercial/buyer-due-diligence.md, and src/lib.rs. No CI/Fuzz/Scorecard/workflow-runner ownership belongs to this feature.

The feature was rebuilt non-destructively at exact 2ff55f0e04a81c4b4efee7c0c910b2cc514582d0, preserving prior head 387a447f856093d02116dfadcf2c4a4a63c6d3ba and live protected main as parents while constructing the merged tree from protected #159 truth plus only those four unique feature blobs. The PR is retargeted to main. No force push, destructive rebase, workflow ownership copy, or valid feature delta loss occurred.

All predecessor check/review conclusions are historical after this real base repair. Current exact-head repository/security/coverage/central review/thread/governance evidence must be reacquired. Keep Draft until one unchanged exact head is terminal-valid; no self/model approval, feature-branch final merge, routine bypass, gate weakening or predecessor-evidence reuse.

[coderabbitai]

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