Persist filesystem WAL writer-epoch fencing across host restarts

[flyingrobots]

Defect

FilesystemWalStore::open reconstructs no writer-epoch state. External-action hosts therefore reacquire a static epoch with fixed fencing, process, host, and lease identities and no previous-epoch linkage on every process invocation. The store validates only in-memory epoch state, so durable request/claim/settlement processes do not prove fresh fencing or a persisted epoch chain.

Observed by flyingrobots/hello-echo#24. Blocks flyingrobots/hello-echo#11.

Acceptance

• Persist writer-epoch acquisition and closure evidence in the filesystem WAL contract.
• Recover the latest closed or active epoch and final commit evidence on open.
• Provide a public host path that derives a fresh epoch, monotonic start LSN, new fencing and lease evidence, and exact previous-epoch linkage.
• Refuse stale, duplicate, overlapping, or chain-gap acquisitions before append.
• Exercise request, claim, settlement, recovery, and replay across separate processes.
• Preserve the generic external-action protocol and application-neutral production vocabulary.

Hello Echo must consume the public contract rather than reconstructing epoch state.

[coderabbitai]

🔗 Related PRs

#598 - feat(warp-core): add filesystem WAL failure atomicity [merged]
#670 - Add trusted causal anchor admission [merged]
#683 - Execute admitted data-only operations in Echo [merged]
#688 - Route executable-operation Actions through scheduler-owned Ticks [merged]
#704 - Add durable external-action settlement protocol [merged]

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