Spike: Prove a representative hard EF Runtime persistence slice

## Question
Can an EF Runtime slice preserve the hardest timing-independent Groundwork semantics across the four providers?

## Dependencies
ADR #1666; MySQL, transaction and migration findings sufficient for the chosen slice.

## Scope
Choose a thin vertical slice containing several of conditional claims/leases, fencing, compare-and-delete, idempotency, checkpoint/outbox atomicity, crash recovery, concurrent callers, SQLite contention and provider-specific conflict handling.

## Completion criteria
Executable four-provider evidence, failure/recovery findings, refined Runtime architecture and issue/estimate breakdown. This spike must not silently become the 32-unit production port.

Start with ADR #1666 and the existing MySQL, transaction, and migration findings, then choose the thin vertical slice described in the issue. Produce executable evidence across four providers, record failure and recovery behavior, and refine the Runtime architecture and follow-up issue estimates without turning the spike into the production port.

Written by the indexing model from the issue text.