Draft: xmr cleanup

Summary

Remove unused XMR signer scaffolding:

• Remove the permanently disabled plaintext key-export route, handler and response type.
• Remove ignored registration spend-key/source fields; registration needs only public vault/group/address identity and birthday. Simulation keys are unchanged.
• Delete unused ungated signing cleanup and broad cache invalidation. Keep durable-cleanup-gated and session-scoped production methods.
• Simplify in-memory cached responses to a required Instant; no serialization or optional timestamp.

Retains DKG, encrypted bound recovery, Prepared/ShareIssued ownership, exact transaction verification, collision accounting, scanner continuity and all timelock checks. Adds/updates public-only registration, removed-route and scoped cleanup characterization tests. Old export URL now returns router 404 instead of the always-disabled handler error; no working export is removed.

Validation and release status

Draft: local tests, builds and CI were explicitly paused by the author and have not been run for this cleanup. Formatting, diff hygiene, repository-wide call-site checks and independent static review completed. This is not a production-readiness claim. GitLab CI may run automatically; merge only after required checks and review.

No image pins, dependency versions, activation parameters, durable state schema or consolidation retry policy changed. These commits are isolated directly on organization develop, not the consolidated security-review branch.

Companion Thornode cleanup: thorchain/thornode!5057 Does not include the unsupported-lock deposit patch in !71 (merged).