ci(comparator): use the bounded Mathlib cache bootstrap #1786

Comparator still lets lean-action perform its strict, single Mathlib cache fetch. In #1756's original Comparator attempt, TLS/curl-55 failures affected 98 downloads and the contract/negative-control step was skipped. This is a bootstrap failure, not a failed proof check. The separate retry of that historical run is not part of this implementation.

Bounded follow-up under #1197: wire the existing, unchanged scripts/cache/get_mathlib.py --lean-action into Comparator, disable only the action's built-in Mathlib fetch, and retain bounded optional-cache fallback before the existing remote restore/contract checks. Preserve all reviewed contract and negative-control commands, sandbox setup, permissions and dependency/action pins. Add small consumer-wiring/dependency-trigger coverage and update the helper's consumer documentation if needed.

Guide already disables the built-in fetch and uses get.sh; do not change it or its URL mode. #1730/#1775 completed their agreed Lean-CI/API/shared-helper scope and remain closed. This follow-up neither activates the #1197 study nor addresses the separate #1726/#1727 API-memory/pin work.

Acceptance: meaningful wiring tests and existing helper regressions pass; exact frozen revision passes all six default gates; independent outside-author-fleet operational CI/provenance review and canonical hosted CI pass before coordinator integration. No Lean declaration, abstract or renderer-source impact is expected.

Duplicate issue/PR search was coordinated with /root/review_list_foldl_max; no dedicated active consumer task exists. Worker /root/mathlib_cache_retry_1730 will claim a separate source-only branch under coordinator /root. AI assistance: Codex scoped this from the recorded bootstrap failure.