alpenglow: add opt-in signing reservations and ordered history persistence

[7layermagik]

Consolidated into #288; no separate merge is required. Its commits are preserved in that replacement branch.

Separate signing persistence and restart safety from certificate/transport performance for focused review.

• Keep detailed vote history and introduce opt-in durable signing reservations with ordered history persistence.
• Persist the reservation before permitting covered signatures. After an unclean restart, a readable history file is insufficient proof that all signed decisions survived; recovery must respect the durable bound and verified-finality gate.
• Keep leader signing and vote signing within the reservation contract. Disk/persistence failures must not authorize additional signatures.

This is a draft because crash recovery is a separate safety boundary. Review the maintained reserved-vote-history documentation alongside the tests. Based on certificate/delivery; retarget after that prerequisite merges.

Validation: complete race suites pass for alpenglow, consensus, node/config commands and blockprod. Vet passes in the combined build. These tests do not constitute exhaustive power-loss or mainnet conformance validation.