Extract PRISM vardiff and block finalization

[kiwidream]

Position in the stack

PR 6 of 9. Base: prism-share-submission. Depends on PR 77. Next: PR 79.

Review this PR against its configured base. See the stack guide for the complete order.

Why this slice exists

Vardiff and accepted-block finalization are separate state machines with strict rollback, durability, payout, audit, and notification ordering. Both were embedded in the coordinator. This slice gives each an explicit owner and names the finalization phases so failures and latency can be observed at the correct boundary.

What changes

• Move share-window retargeting, idle sweeps, speculative difficulty state, rollback, bounded workers, and metrics into vardiff_service.
• Move accepted-block landing and confirmation into block_finalization as six replay-safe named phases.
• Replace large coordinator bodies with explicit ports, compatibility descriptors, and thin delegates.
• Add phase/interarrival metrics and direct tests for the new boundaries.
• Record the evidence-based B3 decision to omit a second finalization lane.

Why the diff is sizeable

The roughly 2,480 additions and 1,580 deletions represent two large coordinator state machines moving into owner modules: about 1,040 lines for finalization and 910 for vardiff, while nearly 1,600 lines leave the coordinator. New direct owner tests and the B3 decision record account for most of the remaining additions.

Review vardiff speculative-state restoration first, then the six finalization phases and their ports, and finally verify the coordinator methods are only construction or delegation.

B3 decision

A second finalization lane is intentionally omitted. Available timing evidence does not show valid-candidate delay, while another lane would require a new durable handoff plus additional parent, admission, and shutdown ordering. The new metrics define evidence that can justify revisiting the decision later.

Validation

• 117 focused vardiff, finalization, and candidate tests passed at this boundary.
• Cumulative PRISM and Rust regressions pass on the final tree.
• Docker image validation was run at this milestone.
• git diff --check passed.

Operator impact

No migration or required configuration change is introduced by this slice.

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Note

Medium Risk
Large refactor of payout, audit, and ledger-confirmation ordering on the block-submitter path; behavior is intended to be preserved but the moved finalization logic is security- and durability-sensitive.

Overview
Extracts vardiff and accepted-block finalization from PrismCoordinator into dedicated owner modules, leaving the coordinator as lazy construction plus thin delegates.

vardiff_service now owns share-window retargeting, idle sweeps, bounded idle workers, speculative difficulty rollback, and Prometheus idle metrics. Coordinator vardiff APIs delegate through _ensure_vardiff_service() and compatibility descriptors that migrate existing embedder state.

block_finalization introduces BlockFinalizationService with six ordered, replay-safe phases (admission → land/confirm → CTV/credit → evidence → audit publish → accounting). The former monolithic submit_block_candidate / _land_and_confirm_block_candidate bodies move here, with phase timing and candidate interarrival metrics exposed on the metrics endpoint.

docs/prism-coordinator-refactor/b3-decision.md records the decision to omit a second finalization lane, using the new metrics as evidence to revisit later.

Adds focused unit tests for finalization phase ordering and vardiff service boundaries.

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